When this happens to me, which it does a couple of times a year, I unthread it, take the bobbin out, and take out the plate. Then I get my bottle of rubbing alcohol and q-tips, my vacuum attachment and I clean everything. Then I oil it and put the plate back on, followed by rethreading, and putting the bobbin back in. I have found that sometimes it was how I was holding my mouth or something when I was threading it. The rest of the time it has been a itty-bitty piece of thread or just some fuzz built up. Either way, completely taking it apart and putting it back together fresh and clean (and oiled) seems to fix it. Good luck!