I remember having this issue with my fwaa and an H10, and the issue ended up being solved by some info in this passage. As it turned out, I had unscrewed the tailcap and rescrewed it without holding the light vertical, messing up the button.
Most FW3x issues can be resolved by following these steps exactly as written:
Unscrew the head AND tailcap completely. (Hold the light vertically, head side up, tail side down. )
Check for any metal burrs on the threads.
Clean the threads and contacts with rubbing alcohol, then reapply lubricant to threads. You can skip if the light is new/clean.
Screw on the tailcap & tighten as much as you possibly can (by hand). (Hold the light vertically, head side up, tail side down. )
Screw on the head. (Hold the light vertically, head side up, tail side down. )
Please see the FW3A Troubleshooting Guide for more. (FW3A Troubleshooting / FAQ)
Lastly, see the quickstart & useful information thread here. (FW3A Useful Information.....)