It may be as simple as turning the bobbin over so the thread comes from the opposite direction. I find my old Singers (1930's) are not as fussy as the new machines. Also sanding the edge will leave non-visible burrs; I would caution against that. I always get the "Singer" brand and have never had a problem. My new Vikings have bobbins that can only go in in one direction, eliminating those problems. Also clean your bobbin area, it may have something in there you can't see.