A heat gun will give you an undetermined amount of heat, as opposed to a fixed heat setting on an iron. With a gun, it's possible to overheat the fabric and cause it to pull away from the structure. This may be what's happening. If the fabric has pulled away, you might be able to pull it back while using an iron to tack it back in place. If you've trimmed the edges, you may have to recover. Also, make sure you are using plenty of fabric adhesive such as Balsarite or Stixit. Coverite does not stick well to bare wood.

Paul