I second HeavyArty's comment. May I also add if they are by Airfix, or Armies in Plastic, Barzo, Conte, Zvezda, or other contemporary plastic makers, his observation is appropriate, too.
Have you cleaned them, before painting? I would wash them in warm water with a couple drops of dish detergent, or a de-greaser--I use SuperClean. I use an old toothbrush to scrub them very gently, and then I rinse them well and let them dry on paper toweling. When dry, I prime using an automotive primer--I use it for everything, soft plastic, styrene, resin or metal. Then I paint. I use enamels, acrylics and oils, and I have no problems with paint adhereing.
Hope that helps!