Originally Posted by TonnieLoree

Sometimes a metal bobbin will become "out of round", meaning it is worn out. I learned this from industrial sewing. If it is a plastic, the same is true. I would try winding a completely different bobbin using the thread from the bobbin in question so as to not to waste it. Do you have a bobbin in a case? If so, un-screw it and check for lint hang ups in there. When tightening back up, hold the tail of the thread and lightly shake it like you would a yo-yo. The case should not free fall. If it does, then it is too loose and will need to be tightened. Remember to only tighten 1/4 of a turn at a time. One more test is to yank on the thread. If it back-lashes then it is still too loose. It could be that you bobbin case is worn out. A temporary fix for that is to place just a tiny, tiny bit (small raindrop; not even as big as a head on a match stick)of vaseline on the side of the bobbin that goes into the case. This will provide temporary resistance until you can get a replacement case.

I hope I explained it good enough for you. If you have a drop in bobbin, you can also use the vaseline method.