When you buy a game, you could be paying the developer directly - in which case there might be problems with the developer's PayPal accounts (they didn't set it to a business account or don't allow payments from certain countries). This is most likely why you're having problems with a particular game and you should maybe write something in the game's comments section or contact the game's support.
Or you could be paying Itch.io directly - and then itch.io pays the developer afterwards. This is most likely the games on which your payment went through without any problems .