dickbaker - 3:33 am on Sep 26, 2011 (gmt 0)
I've run into that quite often. A charge goes through, then not long after (within an hour maybe) I try to charge the card again, and it's denied.
Transactions are monitored by computers, and they red flag anything suspicious, and will put holds on the cards until the suspicion is cleared. Two charges from the same merchant in X minutes or hours can be a red flag.