Apple Pay keeps getting declined at the till

A few things to run through. Open Wallet, tap the card, look at the latest transactions and the decline reason, sometimes it says it plainly. Make sure you're authenticating first, double click the side button and let Face ID confirm before holding it to the reader, a lot of tills time out if you hold it up and then wait for the face scan. Check the card in Wallet is set as your default under Settings > Wallet. If all that's fine it's the bank's side, they hold a separate token for Apple Pay and it can be flagged independently of the plastic, a quick call gets it reset.