Put in a new needle (one recommended by Brother) and make sure that it is inserted all the way in the shaft and tighten securely. Sometimes a bent needle will hit the bobbin case. You can also take out the case and reinsert it to make sure that isn't the problem. Also, cheap thread breaks easily and old thread drys out and frays and breaks which is very annoying. If all else fails it might need to be checked by a professional.
pddrei, September 2006
