cable isnt disengaging the drive properly? take the cable off lube it inside the sleeve and replace. or just straighten out any severe kinks in the cable...assuming that youve had the handle bend over for some period of time?
As above - which is why its never a good idea to fold the handles on machines with cables.
Depending on the design, when you pull the drive cable, a lever on the top of the rear axle engages the drive......that part rarely fails, but the axles do!