Please check that you have whitelisted Operate's IP addresses within SagePay. 

The IPs are: 

SG               52.77.174.17 + 52.77.162.168
US               34.239.4.76 + 54.84.29.90
EU               52.16.91.236 + 52.49.122.79

For help on adding IP addresses to your Account, please see the SagePay specific guide.