Mine has done that too. On mine the bobbin drops into the case from the top. The bobbin is put on pulling the thread counterclockwise. You put it under the hook and then catch it on the left side. Sometimes it slips out from the left side and just pulls from the center. It won't lock the stitch. If yours is like mine, make sure that it "locks" when you catch it on the left. It happens when I am in automode and not really paying attention, since it is an automatic function in my brain.