Have you done the usual due diligence things of checking for tightness of the threaded retaining ring / collar in the tail cap, or oxidation / crud under that ring / switch assy (which likely means removing it to find), etc.?
Be aware that the aforementioned ring in that PD35 is likely reverse-threaded, per this. Won't say for sure, but be prepared for that possibility, as not all have been done the same way. If in doubt, their CS could probably confirm:
I recently exchanged e-mails w/ the Support people @ Fenix-Store (who responded admirably to my requests, it must be said), and among other things was told that: The PD36R uses a 'conventional' threading for its tailcap switch retention ring (CW to install / tighten), rather than 'reverse'...
www.candlepowerforums.com
If you're beyond that stuff, I'd bet you can get that fixed by Fenix for a nominal charge. I've never had to use them for repair (only tech support with questions), but from what I gather they support their stuff pretty well when repairs are involved.