Are you pulling up the bobbin thread before you start? That will leave a mess on the back. Also, check your bobbin tension. You should be able to smoothly pull out the bobbin thread with gentle pressure or if it's the front loading type of bobbin, you should be able to hold the thread in one hand and the bobbin will slowly drop into the other hand. Also, did you loosen the top tension and increase the stitch length? Quilting stitches are longer & looser than normal sewing stitches.