My belts are wasted too. Tried cleaning the belts, etc.
I finally tried to get inside the housing where the band-spring applies tension to retract the belt, and let me say to anyone else who wants to try;
BE VERY CAREFUL
BE VERY CAREFUL
BE VERY CAREFUL
Once you take the plastic housing off (which says "don't remove"

), the spring can snap out of position. This is exactly what happened to me. The sharp metal band exploded out and sliced up my fingers in several places. Blood doesn't match my interior, so well.
And anyway, winding it and trying to put it back in place is a BITCH and I never was able to do it enough to help the retraction. It wasn't designed to be serviceable. I thought about getting a new band spring, etc... but, jesus. Might as well buy one. I'm gonna check out this Gary feller, thanks SBWallker.