I have problem with mind last week, so what I have done is use 12 volt to test it see it running or not. Afterward, I have break it down and see what happen inside, and I found that the communator of the motor is dirty, which stick with black substance, so, I use solvent and sand paper to recondition the communator, just like the electrical technican reconditioning the big electric motor.

Afterward, I put it all back together, it is running fine now.

may be you can try.