Make sure the driving motor (power on) is turning (without the belt attached). Make sure the brush turns freely when the belt is off (it's probably jammed with wound-up hair, thread, and other debris--remove the brush and clean/lightly oil the bearings). Check the tension of the belt which should be fairly firm--replace if sloppy. Rotate the motor shaft by hand when everything is back together and make sure the brush can be driven without the belt trying to jump off. Hope this helps!