The first thing that comes to my mind is the tightness of the covering prior to paint. Did you heat shrink the Koverall prior to your sealing coats of dope? Koverall shrinks drum tight and having used it on everything from a .10 size plane through giant scale I have never seen it sag regardless of the weather. My planes have seen all kinds of extremes from sitting in my freezing cold trailer to sitting in the sun in 95 degree heat with 90% humidity and Koverall is the only material I have seen that doesn't sag or wrinkle. I love the stuff.

If you did neglect to shrink it you may be able to shrink it still just be careful not to damage the paint with the heat.

Brian