I have the same problem with my lighter-weight kilt. I think it does have something to do with the fabric weight. I was told that it could be remedied by a deeper first pleat, but this isn't the case. Perhaps attacking the back of the pleat with an iron could flatten it out, I've never really thought about it. I'll have to try it. You don't want to hit the front tho, as that would affect the A-line of your apron.