If you've hit it with the heat and it didn't shrink out all you can really do is either iron it down and live with the wrinkles or recover it. The idea is to pull the covering as tight as you can across the airframe, especially at the edges, and then use the shrinking to get it the last 5% perfect. If you haven't put the heat to it then make sure the seam is ironed down well and see if it will shrink out.