I found that pickpocketing only works once per NPC no matter your skill level; So if I pickpocket the Mayor e.g., steal his gold, and close down the loot window, then I am not allowed to try and pickpocket the other items again.
That's a bad idea, and it's just plain silly.
Beyond Divinity did almost the same thing: Pickpocketing always worked on anything, but you could only Pickpocket X times, where X was your level of Pickpocket. But this game doesn't automatically have you succeed. I think? So why are you limited to one thing?