Why is my thread bunching up when I sew?
Your Thread Tails Are Too Short
If the thread tails that come out of your bobbin are shorter than two inches, they may get sucked into your sewing machine when you begin to sew. This can cause thread bunching underneath your fabric.
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.