If a payment was declined, but not reflected in your transaction history, this would mean that it wasn't declined on our payment processor's side. In other words, the merchant didn't send us the request for money. That could be due to connectivity issues or an old terminal.
In cases like this, please make sure the relevant card Security settings are enabled by going to PIN & Security, and that your card has not been accidentally frozen or reported as lost or stolen.
If the issue remains, try making the payment using a different method: chip and pin instead of contactless, or try Google/Apple Pay. You can also use a different Revolut card or create a new virtual card. All this can help to determine if the issue is actually with a specific card or with the merchant.