After a thorough cleaning of all lint and even examining your bobbin case really closely to be sure there are no little tiny thread pieces or lint anywhere, making sure all threading is correct and needle new and good I would tighten the main (upper) needle tension bit by bit. Also, this may sound totally dumb, but make sure your presser foot is in the down postition even when free motion quilting. I, ONE TIME, quilted with mine up totally witless as to what was going on on the back side of my quilt...lots of loops and a real waste of thread. Never did that again.