For me the best thing that works to have all those center seams lay flat is to hand stitch them. I made the six inch star by using the method of Linda Franz and Inklingo. I realize that your stars are machine stitched and you are having trouble with the center laying flat. I am sending you a link:
http://lindafranz.com/shop/diamonds/...star-6-inch/24 to her free pattern. In it she also tells how to press the star so you center lays flat. This is just for your information on pressing. Hope this may help you.