First we would make sure that the surface area of the car is clean and free from dirt and debris. We also would make sure that the car doesn't have any factory coating that some new cars have coming out from the factory. Another thing to make sure of would be that the film itself doesn't have anything on the adhesive side.

If dirt/debris isn't the problem then adding pressure to the decal when applying would help with the adhesion. Adding heat along with pressure should help with adhesion.

Lastly, post heating is a must for all products to help with the adhesion.