There are a few possible reasons:

• Your card may not be activated. If not, please do so via your Shell Recharge | Home or the Shell App. Open the app or desktop version, navigate to the main menu and click on "Shell Recharge Cards". Here you can check the status of your card. If there is a tick next to the card number and it says public charging then your card is good to go. Otherwise, please see How do I order and activate my charge card? – Shell Support on how to activate a card.
• The charge point’s RFID reader may not recognise your charge card if it’s tapped too quickly. Try moving the card slowly across the reader and adjusting the distance slightly. To avoid “card clash,” remove your charge card from your wallet before starting or ending a session. The card uses an RFID chip, similar to contactless bank cards.

If you have tried all of the above and it still isn’t working please contact the charge point operator.