When there is a payment issue, you do need to update your payment information to 'reset' the problem, but it sounds as if you are already doing this.
This article takes you through that If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store - Apple Sup… and links to another article about entering the information.
If the updated information is not being accepted, and all the associated information (such as card address) matches, then you may need do contact iTunes support, or even your card issuer to identify the cause. The contact details for iTunes support can be found in the small "Contact Apple" link on the bottom right of the Apple homepage (ensure that you have the correct locality selected on the website).