It might look like an easy block but you have to be able to stitch that line right down the center accurately. Have you tried drawing a center line? I always stitch on the side of the line (towards the corner of the block) so that I don't lose when pressing the seam. Another way I do them is to actually press the square in half, then position it into the corner, glue the triangle into position with glue-stick and the stitch along the pressed seam.