The paint always sticks for me. Try wiping the door with lacquer thinner on a rag to remove any wax or grease. Then sand with 360 wet or dry paper, fine steel wool or Scotchbrite pad. Wipe again with thinner and paint 2 or 3 thin wet coats. It should look like new and the paint should stay on forever.
Living in the trailer park of sense, looking out the window at a tornado of stupidity.