Baloonatic

Are you using old thread? Is the needle the right size for thread thickness? Maybe burrs on needle or somewhere else on the thread track? Are you going too fast for the type of thread you're using? Is the top tension adjusted properly for the type of thread? Have you checked the bobbin tension as well? Is the entire bobbin area and the case clean and lint-free?? Have you oiled the machine? Also, sometimes putting a drop of oil inside where the bobbin mounts (the bobbin case) before you replace the bobbin can help with broken bobbin thread...just remember to sew on a scrap fabric sandwich before resuming stitching on a quilt to eliminate any oil that might be drawn up with the first stitches