I recently had a problem where plays.tv wasn't hooking into any game.
After doing basic stuff like reinstalling the application, drivers etc. I tried to shut down all programms that could hook into my games (like dxtory, OBS...)
The solution was really simple: The problem was caused by the Windows 10 Game DVR. All you need to do is opening the xbox app (search for it), go to settings -> Game DVR -> Record game setttings and screenshots using Game DVR -> turn off
I hope this helps some people :)
(dunno if this is the right place to post it, please tell me where to go next time)