Why does my bobbin thread keep knotting up?
If your sewing machine thread is not properly threaded, the bobbin thread won’t be pulled up into the fabric the way it needs to be. Occasionally the upper thread can catch on a moving part or get stuck, which impedes the easy flow of thread through the needle, creating a tangle.
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.