Sorry to come so late to the party...

Oxygen therapy should work every time to abort a CH, If:

1. The flow rate is high enough...  15 liters/minute is the absolute minimum flow rate although a few CH'ers are fortunate that flow rates as low as 8 liters/minute will work for them, but then flow rates ≤15 liters/minute will have trouble aborting anything ≥ Kip-6 and will likely fail at any pain level ≥ a Kip-8. 

It takes a flow rate of 25 liters/minute to hyperventilate for reasonably short abort times and higher sustained flow rates up to 40 liters/minute will work even more effectively with the shortest abort time possible all the way up to Kip-9.

2. You start early at the first indication a CH is coming...  Most CH will jump at least one Kip-level in pain during oxygen therapy even at the higher flow rates.

3. You start asap if the CH hits while sleeping and catch it before it hits Kip-9

4. The exhaust and inhalation check valves on the non-rebreathing oxygen mask are functioning properly.  A faulty inhalation check valve at the bottom of the "T" manifold will allow exhaled breath to enter the reservoir bag...  At that point you'll have a "Re-breathing" mask and will start building unwanted CO2 in your arterial blood as you re-breathe your own CO2 with every breath.  If that happens, an abort will be nearly impossible.

5. You're not suffering from a low arterial pH that increases vasodilation, or you're not experiencing some kind of allergic reaction that's triggering increased neurogenic inflammation in and around the trigeminal nerve.   

You can measure saliva pH as an analog of arterial pH, but first you need to establish a baseline during a "normal" part of a CH cycle where oxygen therapy is working properly by taking three measurements a day and average them for a few days. 

There's no way to measure neurogenic inflammation without sophisticated neuroimaging, but the experts say it's there during a CH and it's part of the pathophysiology of this disorder.

Nearly all oxygen therapy users who use flow rates that support hyperventilation have had times when oxygen therapy even at the higher flow rates either took much longer than normal to abort a CH, or it took so long it didn't appear to be working at all.

When that happened to me, I started taking a regimen of calcium citrate tablets formulated with vitamin D3, magnesium, and zinc washed down with lemonade.  The thinking here was this regimen would act as a buffer on the stomach's gastric juices elevating them from a pH of 1.0 or 2.0 up to a pH of 3.9 and that would help elevate a low arterial pH...  I would also start on an alkaline forming diet.  See the chart at the following link for details on alkaline forming foods: 
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2 to 3 of these calcium citrate tablets with a glass of lemonade usually brought my oxygen therapy abort times back in the normal range.  Sometimes it took up to 4 of these tablets to get any reduction in abort times.

Last October I began to suspect it was the vitamin D3 that was actually helping shorten the abort times.  I was taking the above regimen and knocking down 4 CH a night in good time with oxygen therapy, so I added 10,000I.U. vitamin D3 and three of the 1000mg. Omega 3 Fish Oil softgel capsules (that works out to ~1000mg. EPA + DHA, the actual Omega 3 fatty acids). 

Omega 3 Fish Oil and vitamin D3 both have a known anti-inflammatory property and there's a list longer than your arm of ailments caused by a vitamin D3 deficiency.  BTW most of us have a vitamin D3 deficiency unless we're roofers, produce pickers, or stay outdoors in direct sunlight a lot without sun block.  See attached pdf file.  It's an article on Vitamin D Deficiency: More on diagnosis and management.  The second page talks about normal and therapeutic vitamin D3 dosage.

The first night on this new regimen I only had two CH.  I was pain free the second night and have been pain free ever since.

We have several others who have tried the Omega 3 Fish Oil and Vitamin D3 regimen and most are reporting positive results.

Getting back to the oxygen flow rate...  and why higher oxygen flow rates work much more effectively and with very short abort times.

There are two parts to successful oxygen therapy.  Hyperoxia - Increasing the blood hemoglobin's oxygen saturation above normal to 100% and hypocapnea - reducing blood CO2 levels below normal through hyperventilation. 

Hyperventilation casts off CO2 from the lungs faster that the body generates it through normal metabolism.  If you're able to hyperventilate long enough and maintain this level of lung ventilation, you'll push your system into respiratory alkalosis. 

You'll know when you reach this state when you start feeling the symptoms of paresthesia - a slight tingling or prickling of the fingertips, lips, or back of the neck.  You may also experience a slight dizziness.

These are good symptoms and the best indication you'll get the fastest abort possible while breathing 100% oxygen as respiratory alkalosis and hyperoxia stimulate the vasoconstriction needed to generate the abort much faster than just hyperoxia.

With that in mind, you have two options if you're stuck with a regulator that tops out at a flow rate of 15 liters/minute... Modify your breathing procedure and technique or get a bigger reservoir bag for your Clustermasx™ like a 40 gallon trash bag or a 55 gal drum liner.

I was working on a new oxygen therapy procedure before my CH went into remission.  The purpose of this new procedure was to help folks like you who were stuck with an oxygen regulator that could only deliver a flow rate of 15 liters/minute. 

The basic procedure involves hyperventilating on room air for two to three breaths while the reservoir bag fills, then inhale a lung full of 100% oxygen, then keep repeating the basic procedure until the abort. 

There's no real time limit on breathing 100% oxygen at sea level pressures.  I've been on 7 hour flights flying Navy fighters breathing 100% oxygen the entire time with no ill effects and I've over 3000 flight hours flying jet fighters and all of it was spent breathing 100% oxygen.  We would need to breathe 100% oxygen for 10 to 12 hours without a break breathing normal air before starting to feel the symptoms of pulmonary oxygen toxicity.

I found this procedure produced a faster abort than just breathing 100% oxygen at 15 liters/minute and it was almost as fast as breathing 100% oxygen at 25 liters/minute.

The breathing technique is important and it requires breathing at forced vital capacity tidal volumes while standing to give the diaphragm a full range of motion.  To do this I exhale forcibly and rapidly with mouth open and jaw dropped like saying the word "Haw" and when it feels like my lungs are empty, I do an abdominal crunch like doing sit-ups and hold the squeeze until I hear a wheezing sound for two to three seconds then inhale rapidly and fully then repeat the entire procedure two or three times while waiting for the 3-liter reservoir bag to fill so I can inhale a lung full of 100% oxygen.  Forced vital capacity exhalation will squeeze our another half to full liter of breath and this end tidal flow is highest in CO2 content.

If I do this breathing technique correctly, I can feel the symptoms of paresthesia by the third exhaled breath of room air. 

Practice it now while you're PF so you'll know what the symptoms of paresthesia feel like when you try the procedure during an actual CH.

Making a super large reservoir bag out of a 40 gal trash bag or 55 gal drum liner is easy.  Fold the open end of the trash bag over at least an inch a couple times and seal with Duck Tape for an air tight seal.  Cut a half inch corner off the opposite end of the bag and stretch it over the 22mm fitting at the bottom of the Clustermasx™ or O2PTIMASK™ "T" manifold then seal with electrician's tape for an air tight seal.

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Put a small plastic bag over the top of the "T" manifold and wrap a rubber band around it to form an air tight seal then turn on your regulator to fill the trash bag with oxygen.  When the bag in nearly full turn off the oxygen and your kit will now be ready for use.

I used this method while on travel one time after getting to my hotel room where the cylinder of oxygen I'd ordered was waiting only to discover I'd pulled the oxygen user's ultimate dumb s#!t and left my regulator at home.  I got the bell hop to get me a large clean trash bag and some electrician's tape.  I tied the open end of the bag around the fill fitting and taped the "T" manifold to the other end, then cracked the valve open slightly to fill the bag...  Joyce shook her head and laughed...  but it worked great!

Hope this helps,

Take care,

V/R, Batch