check the brushes real well, make sure they are not sticking in the brush holders. Sometimes carbon dust will build up inbetween the brush and the brush holder causing it to stick. When the brush leaves the armature surface it breaks the circuit causing the unit to not move.
You can try using motor cleaner and pull on the brush leads to work the brushes loose, then see how it drives.
Also pay attention to how much brush is left, how far down are they worn?
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.