I guess that recognizing volleys isn't that easy. I never had any particular issues with it, but it's also not the shot I care about most. I think a clear movement of your arm has to be involved for the POP to recognize it - just dropping the ball off your racket won't cut it. Especially during a match, that might happen. I had a 2 hour training session some weeks ago where we also did quite some volleys. In the end, the POP recognized 511 FH, 214 BH and 307 Volleys, which seems legit.