I have a Jimi Hendrix Dunlop Wah and I'm getting no volume from it when its on. If I switch it to off everything works fine but nothing gets through when its on. Everything is plugged in properly and I've tried using batteries and ac power.
It's kinda hard to tell on this kind of thing without taking it apart. I'd recommend doing that, and looking at the internals for anything that looks loose or broken.
I had a gcb 95 and it did that when there was no power. So I know you said everything is well plugged but you could check the power alimentation.