I would check the following.
1. Check the end of the spring where it touches the brush to see if it is hitting the brush hood, and getting hung up, therefore not putting any pressure on the brush. Just clip a little off of the end of the spring.
2. Remove the brush spring and slide the brush in and out. Does it move freely all the way in and out? Look for any burrs in the hood.
3. Pull the motor part and cut the comm. I've seen a couple so far out of round that the brushes bounced like crazy unless you pushed on them.
XXX-CR, XXX-T CR, XXX-SCT (Kertez conversion)
Sneaky Pete Racing