Maybe there's a problem with your account. Contact Apple, just in case. Also, you can try logging out of the iTunes & App Store and then signing back in.
Sometimes when that happens, it can ask you to review your payment info. Access your account, through the appstore, scroll to the bottom, hit where your ID is --> view ID and it'll take you to your account info.
Yo, it's only happening to one game that I want. It's happening to the game "Gangstar Vegas" but I tried to download another game from them and it let me download. Idk what the problem is.