from the added description, i'd say that at least one of your strips was too much longer than the one it was sewn to. if you "forced" it by easing, because there isn't another forced seam on the other side of that strip, it has bowed. perhaps some of the individual patches in one or more of the strips aren't exactly the sizes they need to be.
i would recommend unstitching the strips, pressing them flat, finding the off-sized patches and fixing them.