Hi,
Check the tension on your bobbin case. If you go to You Tube there is a video on how to test and adjust your bobbin case. Also adjust your upper thread tension so it is not so tight.
Take a test piece of fabric. Start with the normal thread tension settings, mark on your test piece. As you change each setting, put a line and mark the settings. Go through all the different settings then determine the best setup.
Hope this helps. Keep us updated.
Have a Great Quilty Day!
Pam M