Featherweights can get a piece of thread caught in the bobbin area. There are videos on the net that show you how to remove the bobbin case but it isn't something I would attempts myself. Clean the machines well and check the top tension and thread path. Quite often it is the top causing the problem not the bobbin.On my Featherweight, it is necessary to hold my thread tails as I begin sewing or I will get a thread nest.