Alkaleak take a dump inside the tube by any chance?
Not at all familiar with the light, but my Odepro has a rubber O-ring at the negative end, and what looks like a lockwasher underneath it. Tightening the head squishes the ring so that the negative end of the cell makes contact with the “lockwasher”, biting into it and the bottom of the tube to make contact.
After a hateful little alkaleak did what they all do best, I had to dissolve with AcOH, drill, spin with steel-wool, anything/everything I could do to try to fix it, and it still wouldn’t go. Direct contact to the head worked fine, but it was the “switch” part that was and still is wonky.
So yeah, check the head first so you can rule out at least one thing and concentrate on the others.