There are a number of reasons why your SmartCard may decline. You should contact Services Australia on the SmartCard eIM hotline on 1800 252 604 and they will be able to review your enhanced IM account and tell you why your SmartCard was declined. Alternatively, you can check whether the store has been blocked by searching the Blocked Merchants List.
If you have registered for SMS alerts you will receive an SMS message after the transaction has been declined to show the reason for your card not working.