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.