If you haven't tried this yet, Take the bobbin housing apart as much as you can. There may be a snippet of thread caught in the movements. Also, rethread your machine with polyester thread. Then cut the thread at the spool and pull it through by the needle. Then do it again, sometimes there is lint caught in the tensioner. these are a few of the "tricks" I used to due to free up my dmil's singer 401A all the time.