I've been regularly topping up a child's snapper card (the green one) via NFC using the Snapper android app.

However, trying to top up today, I kept getting messages from the app that the card is an 'incompatible card' and I can no longer top up via the app. Yet the same card used to work ok for NFC top up in the app ... so why would it suddenly become incompatible? Any thoughts?

BTW, my snapper still tops up ok, so doesn't appear to be an app issue.