Is the bobbin tensioning correct? I don't even remember my Mom telling me about bobbin tensioning - so many years ago - when she was attempting to teach me to sew. (I probably wasn't listening...) The manual that came with your machine should explain how to correctly tension the bobbin, or you can look it up on line. Knowing how to set the bobbin tension solves so many problems.
Also suggest that you follow all the other recommendations, as I've had those things happen, too!