Good, that you solved the problem. Show pics when you are finished, please

I was at a quilt shop and they told me to lengthen the stitch and that fixed the problem. Now why didn't I think of that? They also carried a product call "Best Pressed" that is like a starch/sizing product. Everyone was gushing so over it, that I bought some. It is mavelous, but expensive. You just spray it on and iron. I makes the fabric feel like new, and you can spray on as many times to make the fabric stiff like starch. Don't have to worry about scorching or iron buildup. It takes the centerline completely out of fabric. It won't replace starch for everything, but it sure is handy to have around.

The only thing that comes to my mind is: do you have a good tension setting. Try to sew two long (12"-15") strips together. If the piece is slightly curved when you put it on a table it's time to adjust the tension.
Maybe others have another solution

I have always had trouble making this block and I just can't seem to figure out just exactly what I do wrong. I thought starching the daylights out of it would solve my problems, but I was wrong. What seems to be happening is that when I sew the triangle to the corner, that seam seems to "buckle" ever so slightly. This pulls the corners out of square, and pulls up on the center fabric making the square smaller than it should be. So I start out with an 8 inch square of fabric and end up with a 7 3/4 inch square that does not have square corners any more.

I've tried not cutting the corners off of the square before attaching the corner triangle, and it helps some as long as I leave that extra fabric in which makes it hard to quilt and gives a bulky seam. It seems like such a simple thing that I feel silly asking for help!