I pillowcase all of my charity quilts. The trick is to pin corners, first smooth between corners and pin midway on each side. Then, while SID in each quadrant keep smoothing as you go. Then, I do a 1.4" seam all the way around, works best with a walking foot but with out make sure it is smooth between pins and feed in and out, front & back with your hands. I never have a problem. On yours, you may be able to unseam a small way 6-10 inches and ease the fabric as you go and restitch.