I don't starch or prewash....so the quilt police are probably really after me. I also use steam. When I iron clothes, I sometimes use Magic Sizing and then my iron is on steam. Works well. I have used starch on clothes in the past too, with the iron on steam. I didn't like the starch. It left flakes.
I don't think the starch and steam iron should have made the cloth not flat. Maybe you need to experiment on another piece of fabric?
Not much help, am I. Sorry.
Dina