You should cluster the module when you add new module.You can do it like below steps:
1. Login your gateway via WEB;
2. Access the menu cluster(System--->cluster);
3. Click the button of "Automatic Cluster";
You will see the new module be add in your gateway.
Notice: Please upgrade the gateway when you finished cluster,so that your all module have same version of firmware.
Please contact me directly if you have some issues.My contact info be shown as below: