First, double-check that your shipping and billing details are complete and correct, and try checking out using a different web browser.
Common issues to look out for are
1. Your billing address doesn't match the address that your card is registered to
2. Your bank is having trouble authorizing the payment (this can happen for those purchasing from outside of the UK for the first time)
3. The server timed out
If you're still having an issue, let us know and we will see how we can help.
Alternatively, you can try contacting your bank about the payment, especially if you're ordering from outside the UK.