Generally if the mess is on the bottom, it is your upper tension. If you can take off the plate that covers the tension disks, take it off and see if there is thread stuck in the disks. Try different thread, a new needle, Thread the upper tension disks with dental floss and drag it back and forth to see if something sticks like maybe a stray thread. Check all along the way for rough spots. Look over the bobbin section for any rough spots as well. Hope you find out what is causing the problem.