Is it not laying flat because of too much fabric in the corners? If so you can unpick just the corners and sew a little larger seam. If the corners are too tight with not enough fabric then you may need to pick them out and recut bigger corners. Did you stop at the 1/4 inch lines at the intersections or sew across them without stopping. The seam lines that cross really need to be free for the top to lay flat. If you can post a picture I would do that first.