What glue are you using? You need hi-temp glue that goes to @ least 120 degrees or the heat in the sun will cause it to sag again.

Also, are you removing all the original material and backing before applying glue, applying a base coat of spray glue to provide the adhesive something to grip against and are you leaving the adhesive the recommended time before putting the material onto the board?

Here is some of how I did mine...
















I also did my A, B and C-pillar trims as well so that everything above the doorcards matched, similar to newer Fords.