Crispy_Frog, don't forget to check your machine settings. I found that was where my problem was. And, I also made sure to measure and double check each seem. That way you can rule out errors in your seem allowance. Again, because of the machine setting, it put off my quarter inch seam. Also, just re-check you dimensions on your rectangles and squares. They should measure 4.5 inches each unfinished. (When they've been pieced.)
And, remember, we are not all expecting perfect. Close is usable as well. Joining a swap is a learning experience. And, you will learn with each one you do. And, you'll gain more confidence with each one.