I have a Futura which has a plastic bobbin. When the needle goes down in the wrong place. ( whether I am pulling on the sewing or the machine is racing faster than my hoop.) It nicks the edge of the bobbin . Then when the thread is pulled around the bobbin the thread catches on the nick and pulls it back. Then the next stitch catches both threads and so on till you have what is called a "bird's nest." the problem always gets worse unless you can sand the rough spot off or you get a new bobbin. Oh what I would give for a complete metal bobbin. Hope this helps.