Sounds like you're on the right track. Probably one of the motor brushes is stuck or there's a loose/broken brush connection. You probably need to replace the motor if you can't take it apart. Could also be a bad motor winding. If the motor happens to stop with the brushes on the bad winding it won't turn unless vibration moves it slightly. Connect an ohm meter to the plug contacts and turn the blower slowly. The resistance should be low usually <10 ohms and not vary alot.