If your kilt is 21" long then the fell should be sewn down 7". Also if you had more pleats it would help, I'm guessing you have between 10-12 pleats across the back. For a person your size I put in 16-18. The hip pleats should be double or triple the normal pleat depth, mine range from 12-15" that gives a bit of ease.
Also the pleats may not be deep enough and that too could be the cause.
Most companies seem to build contemporary kilts from 3.5-4 yds while some of us put in close to 7.
It's all in the construction.