It is probably caused by dirt and/ or corrosion. Place some 3 in one oil in the hole between the rubber wheel and bobbin holder. Move the metal bobbin holder with your hand or vice grips (with cloth in teeth to protect the metal). Massive amounts of oil may be required. These Singer parts do not wear out but do need oiling from time to time. Good luck. Also there is one screw that holds the belt cover with winder on, you may just replace the whole unit if it has done any time in a corrosive environment.