There are three things to consider (I use channel 1 and FX A as example):
1) On the Main level raise fader 1 to unity, set Main output to an appropriate level
2) Go to FX A and raise fader 1 and the FX A master send to unity
3) Go back to Main level and look for FX A Return (the fader after channel 32) and raise the fader to unity; make sure that FX A Return is routed to Main l/r
Now you should hear plenty of reverb...