Luftwaffe,
It's actually a little bit of both problems. It's partly the way the planes are covered, and it's also partly the way you are trying to shrink the covering. When you get an ARF in before you put heat to the covering you need to go over ALL of the edges and seams with a covering iron to ensure that they are completely sealed before you try to shrink the covering. If you don't do this the covering will pull away from these areas as the covering starts to shrink.

The other part of the problem is the ARF itself. Many times these will get put together in a hurry and the person making the ARF doesn't seal down the edges well enough. Another problem with ARF is the climate conditions during shipping can cause covering to loosen or sag.

Hope this helps

Ken