Just to make sure; Does the pulley on the motor end turn? Does the hand wheel turn freely and effortlessly by hand? Needle moves up and down as it should? A belt can be too loose and too tight, there should be a bit of slack, just enough for the rubber to grip. If it's something related to the internals of the motor it's usually worn carbon brushes. Regardless of how dry the bearings are, they usually turn fine but much better after greasing or oiling. The pulley or motor axle can some times gum up, and if you force the motor pulley by hand it might free up.