Verify that you are using the proper bobbin for your machine and that the thread is wound uniformly/smoothly on the bobbin. Check that the bobbin is installed facing the correct direction and the thread is threaded properly in the bobbin tension.
Also, confirm that the bunched thread is actually bobbin thread and not the upper thread creating thread nests.
Bunching of thread under the material is an indication of insufficient tension of the needle thread increase the needle tension until the threads meet at the center of the materialbeing sewn
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Verify that you are using the proper bobbin for your machine and that the thread is wound uniformly/smoothly on the bobbin. Check that the bobbin is installed facing the correct direction and the thread is threaded properly in the bobbin tension.
Also, confirm that the bunched thread is actually bobbin thread and not the upper thread creating thread nests.
AVOID old or bargain bin thread.
Here are more ideas:
Help Thread keeps bunching up under the plate
How to Avoid Bobbin Thread Bunching While Sewing
Sewing Machine Thread Bunching Up Under Fabric
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Bunching of thread under the material is an indication of insufficient tension of the needle thread
increase the needle tension until the threads meet at the center of the materialbeing sewn