If the motor moves a fraction of a turn and sticks, I would look for debris jammed in the spur gears on the side of the power truck. You can even try turning the problematic motor's armature by hand one pole at a time. Don't force it. If there is a mechanical bind, you'll feel it!
Pat's advice to check for hardened grease is sound. The binding could also be caused by a split worm wheel in the truck block. If you separate the motor from the truck block to regrease, wipe some of the old grease off and look closely with a bright light for any hairline cracks. Good luck, and please post back with what you find.
