You're lucky to be able to use your card at all.
In most of the British McDonalds restaurants, the blue box is on display, but the card is repeatedly declined by the machines and the staff then have no idea what to do. Nor do the managers.
I wrote to McD and AX about it. McD apologised for the inconvenience and said it was a system incompatibility that was being investigated. Amex didn't reply at all.