My backing is one solid piece of fabric......no seams. I am careful in loading by walking the length after every couple of turns to make sure there are not wrinkles or bunching on the roll. If I do happen to seam the backing, I also make sure the seam lays on the roll evenly. I am thinking my problem is in the bars. Although the last quilt effort appeared to have extensive sagging, the final result was only off by about 1/2" because I started "working" it back into shape. So.......if the problem is in the bars, how do I correct it? I didn't put the table/bars/leaders together. Also, I have a friend who frequently 'helps' me load her quilts when I do her quilting and I have to remind her often to not lean against the backing bar; could this also be a factor? Thanks for all your input.