Miriam, I've had that happen, too, mostly with the "CHEAP" metal ones. The only time I had a problem with the Singer plastic ones was when I stepped on one and cracked the heck out of it! I didn't even try to sew with that one! There are some very poor quality bobbins out there -- my son is a stickler - he'll check every one of them before I sew using one! I guess I should be glad he's that supportive, and I really am glad! Glad there was nothing wrong with the machines, just the darned old bobbin!
Jeanette