If your card did not function when you tried to use it at a POS:
Check that your Contactless payments option is well activated for this card.
Check that you haven’t reached your limits of £300 or 300€ consecutive contactless payments. In which case, it will be necessary to perform a transaction using your PIN code.
If Contactless fails again, try to perform the same payment, inserting your card (PIN code).
Check with the merchant that the payment terminal doesn't have an Internet connection problem, which can cause a payment rejection.