[cyclingsarge]

Had similar problem a couple of times. It could be a loose magnet, unbalanced bell, poor electrical connection.

First test electrical connections. Use a continuity gauge. It's a probe with a sharp pointy end and a alligator clip on the other. Usually needs a AA battery. What you do is connect the clip on one end of the circuit and use the probe to test for current. If the connections are good the prone will light up. I recently received a floatr jet from HK with motor and the motor sputtered and didn't seem to know which way to go. Come to find out the bullet connector on one of the leads was not letting current transfer to the wire. So I removed the shrink tubing and heated the bullet with a pen torch to remelt the solder and that fixed it.
The loose magnet need some work and you can google or even search this group or RCU for fixing loose magnets. Same with balancing the motor bell. Good luck.