Just lobbing a few ideas at ya.
You've used a multi meter to ensure power is indeed being fed to the motor not just assuming right? IF yes what power reading do you have?
Check to see if the collar portion of the gear that attaches to the shaft is loose (as in does it spin by itself freely). If yes have someone braze it.
If not a problem:
Pull the board that thing plugs into and reflow the solder joints to the header it connects to. Potential cold solder joint.