Hmm, not 100% sure, but when you cut corner to corner, open and press, how do the corners look? On your practice square, the corner in the extreme upper right looks like it would be perfect (stitching coming out a bit?), the one on the upper left looks a bit off, like the orange fabric comes down a bit rather than hitting the corner exactly. If these corners are a bit off, then when you join them the center is going to be off.