Please check the following as they're often the causes of the problem:
- Check your web browser is up to date
- Clear your cache
- Try a different device or using a different browser
- If the payment page is producing an error, check the billing address you've entered is the same address that your bank, also any authentication notifications that your bank may have sent you
Hopefully trying the above steps resolves your issue. If the problem persists please Contact Us so we can investigate further.