I suggest that your problem has something to do with tension. Remove the bobbin, put it back in, making sure that the thread goes through all the places it is supposed to go. Then for good measure I would re-thread my machine. One of the ways to avoid this problem is to have a "leader", a small remnant of fabric that you sew onto when you are finishing piecing and then something you sew from to begin piecing again. This leader will also save some thread by not wasting it by ending your stitching, pulling the thread out and then cutting. I hope that is making sense.