Is it the bobbin thread that is tangling under the fabric, or is it really the top thread accumulating under the fabric? Usually, it is the top thread creating what is commonly called "bird nesting."
If it's truly the bobbin thread, the bobbin case tension may need to be tightened (search the internet for bobbin tension adjustment instructions), and the upper tension may also need to be tightened to help pull the bobbin thread up.
If it is the upper thread, remove the thread completely from the machine. Keep the presser foot RAISED while threading the upper thread--this releases the tension disk so the thread will seat properly.
Install a brand new needle.
Rethread from the beginning and verify with the manual that it is threaded correctly. Set the upper tension at the midway point between high & low numbers. (This is the factory standard to start from before tweaking adjustments.)
Test. If the upper thread is still coming through too much under the fabric, tighten the upper tension disk. Keep tightening, a little at a time, and retesting. The ideal tension setting is when both the upper thread and bobbin thread meet in the middle of the fabric.
The upper tension is the method to tweak tension whenever changes are made, ie the type of fabric being sewn, etc. Ergo, the tension setting for chiffon will most likely not work on denim because the weight of the fabrics vary greatly. It is important to be able to make upper tension adjustments to achieve the best stitching result for the project. Bobbin tension is very seldom altered except when using special threads that are very light weight or heavy or when the bobbin tension is way out of whack.
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Increase the tension of the needle thread
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