I have a Bernina 640, which has an automatic thread cutter. Most of the time when I try to use this feature, it cuts the thread, but also pulls the upper thread from the needle. I've heard that others have had this problem. Anyone have a tip to make the thread cutter work without unthreading the needle? I contacted Bernina and they said my cutting blade was probably dull (unlikely), but I have had this blade replaced with no improvement.