First of all, you have to know that every game has an own, new and personal activation code. The activation code can only be used for the game you ordered. Therefore you should keep every activation code you get.
If the activation code won't be accepted for any reason, please download the game and then install it again. If this doesn't help, please contact the support team via the Support Center. When doing so, make sure you insert the transaction ID or the bill ID of your purchase so that the support team can be sure that you really purchased the stated game.