I have never heard of this problem before so I just will list of items popping into my mind. What type of batting are you using?? When you stated high loft, I had always been taught decades ago that those types of quilts should be tied. If you said heavy, I would assume that you have used Warm & Natural. I use Hobbs 80/20 and have always used the palm as a rule for quilting. I do all my machine quilting on my Bernina. What type of thread are you using?? I use silk wrapped Mettler. I don't know if you use a lot of starch. Have you try to gently wash the quilt and in very low heat tumble dry?? These are the only suggestions I could think of. I have been quilting for over 25 years and I have never had a stiff quilt. I do HOPE that you are able to resolve this situation. Best Wishes and Hugs !!