Be sure your bobbin thread is threaded from the bobbin correctly. Some self winding bobbins thread right to left and some left to right. Most bobbin cases need to have the bobbin unwinding counterclock wise inside the case. I about went nuts trying to figure out why my bobbin thread was looking like a solid piece of thread after sewing. My bobbin was in the bobbin case unwinding in the wrong direction and kept slipping out of the thread slot in the case.