Hello Sheila,
The most common fault on the 641 is seized bobbin winders, the best way round it is to put a few drops of WD40 on top of the winder pin and let it run down, take an electric drill and place it over the winder pin, switch the drill on slowly and it will free the winder off, put a plastic apron on because of any minute drops of oil that may spash on you, have you made sure the windewr rubber is perfectly dry ?
Ken
drivewithken
October 2010