Tobus,

The problem you describe could be a common one. I do see what may be a similar thing on some kilts that come into the shop for alterations.

To better advise you would you please do an experiment for me.

Hold up your kilt by the two top buckles. You need to watch the area of the Fell or pleats between the buckles in the back of the kilt. Then pull on the buckles. You are trying to pull them apart just as they are when you strap the kilt on.. Do this firmly with about as much force as you would expect to happen while you wear your kilt, move around in it and bend. Say 20 pounds of pull.

Do you see any stretch or movement in the area directly between the buckles? Do the buckles or the tabs holding them to the kilt move at all? Is the stress of the pulling taken up by the stitching in the Fell at all? Does it look like the pleats are being pulled apart exposing the stitches.

If you see any stretching between the buckles the problem may be that the kilt maker forgot to put the stabilizer into the kilt. With the stabilizer in and well stitched down there should be no stretch between the buckles. None, nada, zilch. All the stress should be taken up by the stabilizer. There should be no stress trasferred to the stitching of the pleats.

If you do see any stretch in the area between the buckles when you pull on them the next step is to do much the same thing on the straps.
Grab the kilt near the left side buckle and by the right top strap. Pull about as hard as you did on the buckles. You should see no stretch across the apron. No puckers should develop in the apron or apron fabric. The strap hole should not gape open. There should also be no stress visible where the right strap is sewn onto the apron edge.

If you see any of these things it is a good bet that this is where your problem is. The kilt has stretched by being worn. The internal construction has loosened or failed. Or perhaps is missing altogether.

If you can determine that this is the cause of your problem don't worry. A kilt maker can fix it. It won't be cheap but will be less than a new kilt or the cost of sending the kilt back across the ocean to the original maker.