When my Janome kept popping the thread it was a tiny piece of thread caught in the bobbin. It was probably a leftover from the threadcutter. I think it kept working the needle thread and tightening it until it broke. I finally found it practically standing on my head and giving a poof of canned air. I saw it move and was able to get it off with tweezers. It was really stuck. Yes I know canned air is not a good idea, but was desperate and the dealer is 70 miles away. It worked.