Hello Ram,
For L2 or L3 deployments, the Agent will pop up on the client if "Popup Login Window" is enabled on the Agent and the Agent detects it is behind the Clean Access Server. If the Agent does not pop up, this indicates it cannot reach the CAS.
To Troubleshoot L2 Deployments:
1. Make sure the client machine can get a correct IP address. Open a command tool (Start > Run > cmd) and type ipfconfig or ipconfig /all to check the client IP address information.
2. If necessary, type ipconfig /release, then ipconfig /renew to reset the DHCP lease for the client.
To Troubleshoot L3 Deployments:
1. Check whether the Discovery Host field is set to the IP address of the CAM itself under Device Management > Clean Access > Clean Access Agent > Installation | Discovery Host. This field must be the address of a device on the trusted side and cannot be the address of the CAS.
2. Uninstall the Agent from the client machine.
3. Change the Discovery Host field to the IP address of the CAM and click Update.
4. Reboot the CAS.
5. Re-download and re-install the Agent on the client.
For more information, please go through the following link:
http://www.cisco.com/c/en/us/td/docs/security/nac/appliance/configuration_guide/49/cam/49cam-book/m_report.html#wp1548499