You might possibly be stretching the edges of the pleats when sewing them, either pulling the fabric to keep it straight under the foot or using a thread that is fairly thick and actually stretching the fabric with the extra bulk.

The horisontal creases below the belt mean the fabric is trying to lift up above the hip level - it is a 'fitting fault' which might require a shallow curve at the waist of the kilt which when fitted into the waistband will lift the crease out.

However - it might be worth trying two small darts in the front apron first - although that is a horisontal alteration it can correct the creasing because it releases the tension in the material.

I find that there is sometimes a lot to be gained by tacking or using safety pins at the waist and then fitting the kilt so as to get the pleats to lie right - even if it means having the top edge uneven in order to twist the pleats into place or lift/lower the hem.

With some fabrics the top edge can be sewn exactly level and all is fine, others require a lot of tweeking to look right.