I have a Baby Lock that does that occasionally. I open it up and usually find tons of lint inside. I also will loosen the tension a little. Check the top of the bobbin carrier for a burr...a little rough spot. Check the tension on your bobbin carrier as well. Sometimes over time it can work tighter or looser.
There are lots of things that can cause it. What weight thread are you using? I have to use better quality thread that will withstand the high speed of the quilting machine. I just had to change thread on a quilt I just finished...the quilting thread I started with wouldn't stay together...it kept breaking. I changed to heavier YLI thread and it worked fine..not one breakage.