Try spraying with starch and ironing again. Instead of putting your fabric across the ironing board horizontially (end to end) try putting it vertically (across the width? - small part of your board, the way you have them now. If I have long stips to iron I do it that way with much better results, seems like it takes a little longer (having to move it) but I also get less curve than if I lay it all out on the board and iron the length of the strip. Hope this helps you. BTW your quilt is beautiful.