Hard to do long distance troubleshooting But here are a few ideas that ucan often be the cause -

Sounds like a tension problem - too much tension in the bobbin and not enough in the spool - try tightening the spool tension just a bit - sew on some paper and check the top and bottom to see if the stitches are even on top and bottom and adjust until they are.

If that does not work - try replacing the bobbin with another one and using new thread on the new bobbin - making sure it comes from the same spool of thread. Sometimes bobbins get separated and fit too tightly in the holder. While you are at it, take a brush and clean out any lint from under the bobbin that might be affecting the tension - it's amazing what you may pull out of there!!

These are the two things that cause me bottom thread breakage the most often - hope this helps!!