Not adding flex agent would not cause the paint to lift. Flex agent only (MARTINSR can bakc me up on this) aids in the installation of a flexible, say bumper, to the car when it has been painted off of the vehicle. The problem with painting plastics is this: What kind of Plastic was it? There are many different types. You can just paint them all the same way with the same product. Some plastics have to be treated differently with different products. My guess is that the products you were useing reated to the plastics but that is just my guess
Good luck and Please keep us posted to what you find out.
