debi,
Either your top thread is too tight or your bobbin thread is too loose. Most of the time it is bobbin too loose or debri in your bobbin slot. Try cleaning the little slot that your thread goes through in your bobbin. If it is clean, try tightening the bobbin tension very slightly at the time and see if this corrects your problem. There should be about 1/3 of your bobbin thread showing on the bottom of your embroidery.
bsue