One more tip: Sometimes when you load a backing one side sags. The easiest way to remedy that is to roll the backing back and forth a few times, all the way from everything on the takeup roller to everything on the bottom roller, keeping the backing taut the whole time. You'll see improvement each time you roll it until there's no more sagging. I don't know why that works, but it does. And I don't know why the sagging happens, either, but it does, lol.

If I can I load those with the seam horizontally.

I used to make my seams vertical too and I'd have that bump where the seams met as I advanced the quilt. THen I read to put your seams hortizonal as then your quilt should stay level as your advance. I still have a little trouble when I use leftover blocks in my backings but not as much as I used to. I try to place the blocks in various areas around the backing so it eventually levels out as I advance it.

Another vote for just turning the quilt.

If you can't change the backing to a vertical layout, you could try a small piece of batting on each end. The width of the batting should not be more than the circumference of the bar; keep the length manageable, 6-8 inches should be enough. The batting should be placed on each side at the bottom of the backing just as you start to roll it. This should provide enough "bulk" for the backing to be flat when you reach the top. Just make sure to remove the batting before quilting the last row to avoid getting it caught in the stitching. I also find that it's sometimes easy to stretch the backing while attaching it, especially if it's pieced. Measure the backing side-to-side in three different places, and use the average. Then centre this measurement and temporarily mark your leader cloth, line up your backing fabric, and attach. If there is too much fullness, the backing may need to be trimmed. Good luck.

I too vote for turning the quilt if you can. I did a quilt for a friend that had both horizontal and vertical seams and could not avoid this issue - for that quilt after rolling back and forth a few times I finally used batting scraps

I'd do this too - it has resolved the problem for me several times.

Also, one question I HAVE to ask - were the selvages removed before piecing the different parts of the back? Leaving the selvages in sometimes SEEMS like a good idea, but they really do mess up the tension and get the fabric out of square. If the selvages were left on the fabric, I'd remove them and re-piece these section. THEN do the "roll back and forth several times" thing... of course, once you remove the selvages, the problem may have been resolved.