Slippage of the belt...

[The Wizard of BC]

If you are wearing a Traditional Style Kilt the belt should not be around your naval. It should be at your natural waist which is above the naval.
To find your natural waist try these hints.
Take a 1 1/4" or wider belt and fasten it just under your ribs and just above your "love handles at the sides. Then bend side to side. The belt will want to settle into the narrowest part of your natural waist. This is where your belt wants to settle naturally and this is where it will not move around.
Now put your kilt on and cinch the top straps into the same natural waist hollow. Then put your belt on covering the straps. Even fastened loose the belt will stay put all day.

[Riverkilt]

I've also noticed that just like having to retighten the cinch on a horse's saddle I have to retighten my belt a notch after I first put it on.

So, when I do wear a belt with a kilt I cinch it up, wait a few minutes, and retighten it.

Ron