It is the bias edges that cause this to happen. When you put the eight triangles together did you do so that you ended up with two halves that had to be sewn with one long seam? If you did then you can fudge a little in the center by making the seam a little deeper towards the center of the seam where all the ends come together. Next time when you piece the diamonds together start sewing at the end where they all come together and work towards the outsides of the diamonds. That will leave most of the fullness from stretching of the bias to the outside instead of in the center. With the fullness to the outside you can then gently work the fullness in as you apply the outer squares and Y seam triangles. We'd love to see a picture. Lone stars are my favorite pattern!