What you are describing is an issue with poles that are not ridged enough to hold straight their entire length. One thing I can suggest is shop around for poles made from a different material that will stay straight. My old frame had a dip in one of the rails. Load the quilt the same as you did before, then check the tension across the quilt as you advance it when you quilt. If there's an area that's loose, unroll just a bit, then lay a piece (or two) of scrap batting on the top and re roll. The extra thickness of the batting scraps should take up the slack.