This is absolutely bumfuzzling.
I have, within the past couple of months, acquired a Guitar Hero II Xplorer, and it's a fine guitar, and everything on it works fine - except for the whammy bar. It's not dead or flat, it just doesn't work. Holding the bar down does nothing - it used to, but now it doesn't work. Recently - and I experienced this during a session of Guitar Hero III - the whammy bar is basically working, despite me not holding the strum bar down. As in, I'm not holding the whammy bar down, but every sustain is still affected by the whammy bar. My best guess is that the mechanism inside the guitar broke, but if anyone else has other ideas, I'd appreciate them.