Here is what I see in the picture. The 2 pinwheels with the dark background at the top middle are wider apart than the 2 similar pinwheels at the bottom. This is causing the upper portion to be wider than the bottom portion of the quilt. Unsew the left side and the 2 upper middle pinwheels. Correct the 2 upper middle pinwheels to be the same width as the lower middle pinwheel and sew it all back together. It should be more square afterwards.