First, I would measure the distance between the backing bar and the take up roller all across the frame and make sure that it's the same. If the rollers are joined, make sure that the joint is tight.
The thing that I've noticed is that if I leave part of the quilt under the machine as I roll, that spot seems to get stretched bit. I have started to roll the machine off to the side before I advance the quilt. When the quilt is too large to do this, I carefully measure every time I advance and try and remember to move the machine each time, right, left center, etc.