Sometimes your card issuer can decline your card for any number of reasons.
We recommend trying again or using a different card if this happens.
Our payment gateway is powered by Sagepay and is very reliable. When we see why payments do not go through, it's almost always on the card issuer side.
We also support payments via Paypal.
If you believe your card is good to use, we'd recommend first contacting your card issuer as it may have a fraud alert, which sometimes happens if they detect a series of online transactions.