Are you "touching-up" by applying a very small amount of paint to a very limited area with a small brush tip, or are you over-coating a larger panel by air-spraying the paint? In either case, you'll need to properly prepare the area like Smudger suggests.... degrease, roughen, degrease, prime, and then apply the top coat.

For very small "touch-ups", I have been pleased with the gloss black enamel from Micro-Tools. Apply it very sparingly with a very small brush tip, and let it dry for several days.