I have two brother machines that I have purchased from Wal-mart. I had this to happen to me once and come to find out, I had bought more bobbins off the shelf at Wal-mart and I had bought the wrong bobbin. Have you bought replacement bobbins and could this be your problem, also. I went back and bought somemore and compared them. One bobbin is a little thicker than the other and that is what made all the difference. Hopes this helps......