WB, it looks as if you have a sheet stab? When shrinking the covering on a sheeted surface, (stab, fuse, rudder, wing) you should put some pinholes (one or two) in the wrinkled areas before using the heat gun. What happens is the air inside heats up, and holds the covering away from the surface, then when it cools, the wrinkle comes back. This will also apply to smaller built up surfaces like stabs and rudders, that have little air space in them.

After it fits tight to the surface, go over it with an iron to stick the covering down.