Card declines happen between the payment processor and your bank. Either the bank can reject the payment or the payment processor can decline the payment if it matches their fraud detection criteria. Our experience is that the majority of declines come from the customer's bank rather than the payment processor. Unfortunately if the bank decline the payment we often do not get access to the details of why this has happened.
Our payment processor provides more information on this here:
If your card is declined and you're worried about missing out on tickets for an event while you speak to your bank, feel free to drop us a line. Normally we'll reserve you a ticket at the event for a limited time to give you a chance to speak to your bank. Contact us by submitting a support request through this support site.