I agree with KWhite.. often folks try to piece right to the edge of stars and you can't do that and have them lay flat.. also if you don't piece them in "sections" with the final seams being straight across you get an "outie bellybutton".
I too would take the stitching out of the center yellow diamonds and re-stitch. But I'd do it by hand so I'd have better control of the "poof" and where the seams end.. then if you stop short as K mentioned you can get a lovely swirl to the center of the star and get it to lie nice and flat.. you can also use a little wooden mallet to smack it into submission if necessary..lol <eg>. What a lovely find.