We have been using "Click and Pledge," www.clickandpledge.com, as our registration and payment service for the past 3 years.
Over this time we have a few reports every year of nagging problems with credit card payments. What we have found is that the "gremlin" is most often associated with a mis-match on expiration date, the 3- or 4-digit security code (which appears on the front, not back, of American Express cards), or the billing address affiliated with your card.
We have also found that the payment page on our registration site is far more sensitive to alignment of these details than other online payment sites, even Amazon. A common complaint is that the credit card used for registration payment is routinely used for purchases on Amazon, yet rejected by our registration site. We have also heard from registrants who failed at using a second credit card.
If you find yourself in this situation, we have always found the solution to reside with your credit card company's approval system. As such, we suggest you contact your credit card company, who can see your attempts at payment, and the associated blockage. In doing so, they are ideally suited to direct you to where the "gremlin" lies.
Contact us by e-mail if this solution path does not lead to a successful payment.