Quoted from RonSS:
I'm pretty puzzled by this. Seems to turn fine by hand. If the grease was solidified I would think even hand cranking would be an issue.
When I had it apart, I had the feeling the gear and slotted shaft weren't playing well together. Seemed to turn fine without the armature.
I'm inclined to say the gears are somewhat stripped perhaps. Having a hard time matching up.
Open it back up and take pics so we can see. Please