Another thought came to me there are a couple of different sizes of bobbins, although by mistake I have bought them, filled them up put them in my bobbin case, the machine sewed but not well. If it were a matter of thread and I thought my thread was old and it would have to be fairly old I would get rid of it. Now one thing I do stay away from is the cheap thread, do not trust it. Good luck, what a pain for you, I know little things sure make a difference with machines.