I also have this problem with my Brothers, but it's the top thread being drawn underneath and knotting. Maybe your machine cuts the top too long, though if it's cut too short it won't pick up the bottom thread to sew. I have developed a knack of pulling my pieces out to the left side, while cutting, with the foot barely lifted, and the top thread becomes fixed under the foot. Or try sliding your fabric under the foot sideways instead of from the front, with the foot almost down, so the top thread is caught at the side and held down. That should prevent it from being drawn beneath and tangling. You may have to hold the thread firmly for a few stitches. I also use a knee lift - love it. Hope this helps