It is best to start with a simple thing,when was the last time you cleaned in between the tension discs?Alot of the polester thread is fuzzy & the lint builds up after while.
It's real easy to take the nut & spring off,then the release washer,then take the discs out & look in between them for lint & sometimes there might even be grooves that the thread wore into thereby not allowing them the grip the thread like they should.