Kat - sorry to hear of your situation.  We've all been there in one fashion or another.

Verapamil is the first choice of many headache doctors, but it does not work for all.  Most here who have used it believe the immediate release version is more effective.  However, this is a STRONG drug, and you need to be careful with it, especially if you are at 320mg/day.  When and if you do change to something else, you need to taper off the Verapamil -- don't stop all at once.   There is danger of a blood pressure spike if you come off it too fast.

It worked well for me for several years, and then one day - poof.  No help at all.  You might ask your doc for a prednisone taper which can help to knock the CH back, and allow the Verapamil to take effect.

Best of luck to you.  Others here will no doubt weigh in with some additional ideas.

I was on lower doses of verapamil for years and then it stopped working. In the end I had chronic clusters like an episode every 5 hours. After much chasing my tail I finally went up to 1200mg SR per day and then slowly tapered it down over a period of months. I am currently on 120mg verapamil per day plus 800mg ibuprofen at night and it is working well. Still get the odd shadow but no CH.