Were the original pattern pieces correct?
The "squares" around the center square appear somewhat rectangular in the picture. Are they supposed to be that way?
Sometimes I will do a paper cutout/mockup with the pieces to see if they are right and fit together the way I expected them to.
I had bought some plastic templates (printed on sheets and then one cuts out the ones one wants for a particular block) and the "basic unit" was not square. It was 1/8 inch bigger in one direction. Not a huge amount, but 1/8 inch is quite a bit to ease in on a four inch length. After fighting with the whole quilt, I FINALLY measured the template. YUP. It was off.
That was a few years ago. Now I have a little more trust in myself - and less "complete faith" in patterns and instructions.