I had this happen to me. I was so frustrated. I found out that a piece of my bobbin carriage was broken when I called Singer to find out why I couldn't keep the tension on my bobbin thread. I had to order a new part and wait two weeks for it to come. Happened to me twice after that.
I finally gave my machine away because it was too hard to quilt with. It was fine for piecing, but couldn't handle the quilting part. (Now I am kicking myself for giving the machine away since it worked fine for regular sewing.)