Contact Surefire warrant/repair. They will either send you a new one or have you send in the one you are having problems with for repair. I've had a few items go
Everything from cracked lenses on bezels, tailcaps failing, even a P60L lamp assembly. Surefire sent me new items without question. The only things I've ever had to send them were a couple V70 speed holsters that had broken springs and the P60L. The rest of the time they just sent me new parts.
In the meantime, I would use a Z48 tailcap. Any C-Series tailcap will do, but the shrouded tailcap prevents any accidental activation. And since it's being used with a weaponlight, that's definitely important. The Z48 is also just a beefy tailcap too.