There can be many reasons for your bank to decline a card. We never decline a card directly, it's always your bank that tells us if it was declined.
The most common reason for declined cards when funds are available is because your bank does not feel that it's typical of you to make a purchase like you are for Tweetwall, and that it may be fraudulent. To see exactly why the charge was declined, you should contact your bank and tell them that you were trying to make a purchase, but it's not going through. They will be able to look at your authorizations and tell you the exact reason it was declined. In most cases, they can then make it so that future charges do go through.