2 things immediately pop to mind - both of which have happened to me! Once, I had a TINY, TINY bit of thread lodged down in the bobbin area. I thought I had cleaned everything, but, when I took it apart the second time and used a flashlight and a little mirror, I found this little piece caught. Amazing how much frustration a tiny bit of thread can cause. On another machine, I found a tiny burr on the top of the bobbin holding case. A minute with an emery board and problem was solved. I found it be lightly passing a bit of cloth over what should have been a smooth area and it snagged. Hopefully your problems will have one of these kinds of simple (although insanely frustrating) things.