Uncharted 2 help

I only need the trophy where you have to get 200 kills with the gatling gun, but i can´t buy the gun (or anything) from the single player store. Whenever i try to purchase sth from the store it just says: purchase failed. I have beaten the game on easy and crushing (on the same save file). Also i cant buy anything from the multiplayer store. Pls help.

Also sorry for my bad english.