I have found that sometimes the security code on my credit card under my itunes account falls off.(usually after I have to accept new terms and service from apple)
Try going under settings-itunes store-hit apple ID-view apple ID -go to payment information and put in the security code to your credit card.
When mine is missing, I have to keep logging into my apple ID.
Also if you changed your itunes account after you downloaded the game, delete and redownload the game so it will be under your current itunes account.