Tried to stitch around my quilt top/batting/backing and the bobbin thread is wadding up underneath and in the bobbin case. Doesn't seem like the feed dogs are moving, either. Since it behaves ok when I just sew two pieces of cotton fabric together, is it a problem with the thickness maybe? I compared the mattress pad I'm using with a small piece of regular batting and there is not much of a difference in thickness. Hoping someone can help. Thanks!