Merchant declines can occur for any number of reasons depending upon the vendor. Most vendors will have an automated anti-fraud system that may decline spend for various reasons. If you feel this is the case, you can message email@example.com in order to troubleshoot the particular vendor’s reason for declining you.
Please make sure that your card details match that of your account. A merchant can decide to decline a card if the address or the name of the account is different than the card. They also may decline it if you are using the United States issued card while your account is listed as a different country.
If this doesn't apply to you, please visit this general page for card declines: What If My Card Declines? – dashfi (zendesk.com)