Why is my thread looping 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.
Why does my sewing machine keep jamming underneath?
However certain you are that the problem with the machine is most likely due to a huge tangled mess of thread in the bobbin underneath the fabric, the most common reason for the jamming is usually the lack of sufficient tension in the upper thread.
If your thread is pulled tight on the underside and not forming an even stitch then (counter-intuitively) it’s usually the top thread tension that’s wrong. Sometimes very lightweight fabrics such as sheers can get dragged down into the machine so it’s a good idea to use a straight stitch plate.