On my Gammil machine, two problems;
1) need machine oiled 12 hours before stitching.
2) when replacing needle, because it had a round shaft, to be sure it was in straight would put a pin through the eye of needle to hold in place while tightening the screw.
Once the bobbin case was out of round. Have no idea what happened. It looked perfectly fine. When put in place, the thread pulled off with mixed tensions. Replace bobbin case.