What causes bobbin thread to ball 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 is my sewing machine knotting up underneath?
If the thread has knots, is not smooth, is uneven, or is loose on the bobbin, then it has not been threaded correctly. If your machine uses a bobbin case, follow your sewing machine’s instructions to remove the bobbin from the case and re-thread it.