depending on model it could be a position finger or bobbin stop not set correctly,on top feeder machine there is normally a small plate l/h rear that hold bobbin case and should have a cap of approx 1.2mm for thread to pass thru and also r/h front spring loaded stop with same gap,also put drop off oil between case and hook,front or side loaders use small metal piece that can be adjusted to give just enought gap for cotton to pass thru
johan, December 2012
