Have you taken the needleplate off and the bobbin race out? Sometimes there will be a wad of dust or a thread piece wrapped up inside and behind. Also, have you oiled it everywhere needed, including underneath? You could also have a gob of dried up oil in there somewhere. It could've moved at one point, making it easier to sew, then gotten stuck again.
I'm betting on a loose piece of thread wrapped behind the bobbin carrier, just because it's happened to me before!