If you're using an oxygen flow rate of 15 liters/minute...  I'd get a regulator good for a flow rate of 25 liters/minute or higher...  I use much higher flow rates.

I would also get some pH test strips...  (I use test strips from pH ION as they have a 0.25 resolution, but other pH test strips may also offer this resolution). Follow the instructions that come with the pH stix using the two-day collection chart to log saliva and urine samples to calculate an average daily normal reading.

I've found that if I take a saliva pH reading as soon as I can at the start of an attack and prior to starting oxygen therapy, my pH is 0.25 to 0.40 below my daily normal reading.  It's also interesting to note that when I take a second pH measurement 5 minutes after the abort with just oxygen therapy at a flow rate of ≥25 liters/minute, the average pH measurement is 0.40 higher indicating my saliva pH is back up to the daily normal reading to slightly above.

(I developed an excel data collection sheet that will plot out daily pH readings.  If you want a copy, shoot me a PM with your email address)

If these tests indicate the pH level is lower than normal, arterial blood has shifted from neutral to more acid for some reason. It could be due to diet or some other condition.  Whatever the cause, a low arterial pH will make oxygen therapy less effective as this condition causes vasodilation and that tends to make the cluster headache triggering mechanism more effective. If the pH measurement is well below normal, the level of dilation may be greater than oxygen therapy can handle. As a result, the frequency and intensity of cluster headache attacks will increase to the point where a flow rate of 15 liters/minute can become totally ineffective or take far too long to abort attacks at Kip-5 and above.

I've found my chronic "high cycle" periods coincide with a lower than normal pH so I take the Calcium and Magnesium with Zinc tablets.  

Dennis and Guiseppi are spot on...  These mineral supplements can help regulate the arterial acid-alkaline balance back to normal levels if taken as directed on the label. If that doesn't work, cut back on red meat and cheese for a few days adding more green veggies.

As always, results can and will vary, but taking these tablets usually takes me in a day from a rate of 3 attacks a day during a high cycle (I get hit while sleeping) down to a rate of less than 1 attack every 3 days (usually when I miss a dose).  See your doctor if you're taking thyroid medications as these mineral supplements can interfere with thyroid medications.

Take care, and let us know the results if you try the pH strips and mineral supplements.

V/R, Batch