Possibly because the brushes weren't making good contact with the commutator. Possibly the pole windings were a little weak. Possibly the solenoid coil was a little weak. This could go on for a while, all of them are guesses Keep in mind that rebuilders are trying to do the job as cheaply as possible. They miss things for that reason, the problem could have been any of the above, it could have been a bushing fitting loose on the shaft causing it to bind up.