Waves plugs not passing audio some times in PT10 HD

I'v mentioned this before and never got to the bottom of this. I'm on windows 7, PT 10.3.10 HD and latest waves plugins all genuine. Every now and then some ways plugs don't pass audio, if I make them inactive the audio gets through. I then have to close the session and reopen and all is well. Anybody suffers this before. When I contacted waves they said they had never heard of this and must be a pro tools issue, but I'm not quite sure of that. Many thanks in advance :)