I haven't read all the answers, so I am sorry if I am repeating, but I was having the same problem when I first started quilting. I discover what my problem was. I was making sure that the seams were pressed in opposite directions, but when I butt them against each other, I was only doing it on the edge, instead of all the way down. Your seam is 1/4 inch, so you need to make sure the seams are butted all the way down. It helps to pin on both sides of the corners.