It sounds to me like the tension on the bobbin case is off. It sounds like it is too loose. Try taking the bobbin case with the bobbin in it and threaded the normal way and lay it flat in the palm of your hand. Make sure your hand is out flat. Now take the thread and lift up the bobbin case with it. The bobbin should almost, but not quite, come off your hand. If it comes up off your hand then it is too tight and if it unwinds too easy, it is too loose. The bigger of the two screws on your bobbin case is what adjusts the tension. Good luck. Hope this helps