You'll likely find that these motors are easy to take apart, and terribly difficult to put together - requiring custom tools and a little coaxing.
Step 1: Take Apart Motor
Step 2: Inspection
If everything looks good, take a look at the brushes. They should have a concave portion on one end where they touch the motor armature.
If everything looks good, we'll hope that the brushes just need to be reset.
Step 3: Custom Tools
Push the spring and brush assembly into it's holder.
- use a small screwdriver to push the spring into the holder (there's a channel on the side for this)
- push the brush into the holder while holding the spring back
- insert a bent paper clip through the small hole
Step 4: Assemble Motor
Lower the motor housing onto the paperclipped rear casing. Be sure the tightening screws line up.
Now, pull the paper clips out of the housing. The brushes should gently push up against the armature (you shouldn't here loud clicking or similar).
Screw your motor back together and make sure the shaft spins freely.
Now test. The first time I gave it power, my leads had a rather large spark. The second time it powered on without problem :)