I am not sure if it is the right term but I have a question regarding the under apron on a kilt.

I have made a few kilts now but sometimes have a problem with the end of the under apron on the kilt folding when I walk. Not the entire apron, but just the corner folding on the knee and not always sitting flat when I walk.

Is there a way to prevent this? Do I need to make the under apron longer or tapered?