Are you using the right bobbin? It needs to be a metal 66 class bobbin. If it's a plastic bobbin, is it damaged around the edge? Are you using good thread? You never want to sew with "hand quilting" thread in your machine. Is the spool of thread moving freely? If not, put an empty bobbin underneath it. Is your tension release working? When you lift the presser foot, does the upper tension release?