GuitarHero controller upstroke wont work!?

Okay, im about to smash my GH3 controller into pieces, after using it for GH2 and GH3 it suddenly stopped working, only downstrokes tought. So i have no idea how iam supposed to fix this, and i would very much appreaciate your help! The guitar does work but the downstroking wont react, it does react sometimes but otherwise the downstroking is as good as dead. Please guys i want to play this game again!