I may be wrong, but in an earlier post about your kilt pleats, it looked like your kilt was made of very light material. I remember some saying you may have a hip dimension issue with your kilt. Having the hip measurement too large will cause the material to hang like curtains with waves in it as you are forcing too much material below a set waist circumference. The material has to go somewhere, so it folds back in on itself in waves. Having too light weight material can have a similar effect, as the weight of the material will not pull down on itself enough to resist the lateral wave potential. Either of those issues or, worse yet, both issues combined and you have a problem no amount of pressing will cure.

Just want to be certain you are trying to fix the right problem.

Best of luck!