Why is my thread bunching underneath?
A: Looping on the underside, or back of the fabric, means the top tension is too loose compared to the bobbin tension, so the bobbin thread is pulling too much top thread underneath. By tightening the top tension, the loops will stop, but the added tension may cause breakage, especially with sensitive threads.
How do you adjust thread tension?
Changing Bobbin Tension
Turning the screw a tiny bit counterclockwise will loosen the bobbin tension; turning it clockwise will tighten the tension. Some quilters keep two bobbin cases on hand, leaving one as-is from the factory and using the other when they need to make adjustments.