I was wondering if by chance you'd miscut the square! Glad you figured it out ... and hope you didn't have too much wastage.
A hint ... I oversize the large square (instead of the 13-1/2", I'd cut 14-1/2".
Then X-cut and stitch all triangles in place. Then cut off the excess so you can get the edge of the quilt nice and straight!! (then be sure to leave the 1/4" seam allowance, not cutting too close)