Be sure you are using the correct bobbin for your machine. Even though it appears to fit the bobbin case, if it is the wrong bobbin, it could cause major headaches.
Try a different bobbin. Try different thread.
Also check that the bobbin thread is wound smoothly and turning the correct direction in the bobbin case. Make sure the thread is threaded through the bobbin tension properly.
AVOID old and inexpensive, bargain bin thread!
There could be a nick or burr somewhere that is catching the bobbin thread and breaking it.
Try loosening the bobbin tension a bit. It could be too tight, especially if the bobbin thread is pulling the upper thread down to the underside of the fabric.
There could also be something wrong with the bobbin case itself that may be resolved by trying a new bobbin case (or borrow one from a friend, etc.)
Dealing with Lower Thread Breaking Sewing
Bobbin Thread Breaking How to Loosen Bobbin Tension
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