I usually don't use steam when I'm pressing my quilt fabric wether I use starch or not. It just depends on what I'm making. I think it's more a personal preference than anything else. Steam will stretch the fabric, so you want to be careful when working with bias edges.
I don't have a clue why some fabric won't get stiffer when using starch and others do... but I have noticed it myself. I usuallky just put more starch on that fabric, until it is as stiff as I want it to be. I would rather use more starch since it's so cheap, than to try and sew with fabric that isn't startched, and ruin something because it stretches.