Zpainter as Chip Mull said it is risky to use a heat gun on polyester as there is the chance of overheating areas in the fabric. Once that happens the fabric in that area will relax no matter what and allow the taut areas around it to wrinkle.

I have noticed from time to time that polyester will shrink when cooled and do as you have described when it heats.
I have also noticed that even though the new non taut dope formulas shrink very little that over a period of 60 to 90 days the fabric does get tighter and the wrinkles fade away.
Disappointing though it may be right now, I would wait for a couple of months before I tried anything drastic.
For the modeler Koverall is still a great product and is usually happy with just about anything that you put on it.
The one thing that I would have done is use Stixit rather than Nitrate to attach it as it has non tendency to soften when sucessive coats of nitrate have been applied allowing the fabric to creep.