I am assuming that you are using the stitch and flip method. You draw a diagonal line on the 2 1/2 inch square piece, sew along it and trim. Very accurate drawing and sewing is needed. Instead of sewing directly on the line, try sewing beside the line, on the side to be trimmed. Also, before trimming, gently press the seam and make sure the wing that you folded up covers properly. I have been known to draw my seam line on slightly too small blocks. That gives me a visual reference when sewing blocks together.
It would be a shame not to use what you have already cut.