Currently Being ModeratedJan 31, 2013 1:20 PM (in response to Jasongreeen)
First try logging out and logging back into your account on your phone. If that doesn't work, try going into your account settings on either a computer or your phone and removing and adding your credit card again.
I've had this issue before and usually, one of those two has worked.
You may want to try checking your bill; sometimes a random purchase or charge will fail to go through and the card will be marked as unusable.