thorsten_iceland - 7:16 pm on Jun 8, 2012 (gmt 0)
What payment gateway are you using?
And what were the specific decline codes you were given by the gateway for the legitimate customers?
For example: 05:Do Not Honor, or 51: Insufficient funds...
(If you can't see the specific decline codes in your gateway back-end, you may need to contact them to find out how to get that information.)