why wont paint stick,

I've never used Humbrol, but in my experience, using metal model paint as a primer doesn't work too well... color coats flake off too easily when removing subsequent masking.

Try putting some dull coat on first. Just a suggestion. If you just want to do some scraping of the silver showing through this should still work.

thanks for the ideas guys i will try them,and yes i want to scrape parts of the metal cote off for the weathering of kit kit.

Or try some primer. I got me a spraycan of autoprimer (white), which can serve as a base for the next layer.
If all doesn't work, take some 1000 - 2000 grit sanding paper and roughen the surface gently. The smoothening of the coat can be done afterwars with some gloss varnish / future.