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I hate when this happens! When the bobbin thread is bunching up it typically means there is a problem with the needle thread. I know that seems backwards, but as long as the bobbin is wound nice and smooth and inserted into the bobbin case correctly, there isn’t much that can go wrong there.The first thing I would do is re-thread the machine. Make sure you do this with the presser foot raised. Raising the presser foot releases the machine tension on the thread and allows you to easily pull the thread through the machine.
Doing this also allows you to feel how much tension is on the thread. If the thread is bunching up on the underside of the fabric it usually means that there isn’t enough tension on the top thread. Try adjusting the needle tension and running a few more test stitches.