Might have to do with your IIS settings.
http://service1.symantec.com/support/ent-security.nsf/854fa02b4f5013678825731a007d06af/6bc393d0d2f824e088257386006a2b8f?OpenDocument
Client Computers Fail to Appear in the Symantec Endpoint Protection Manager After Deployment
Question/Issue:
You successfully deploy Symantec Endpoint Protection (SEP) to clients in your network but find that they do not appear in the Symantec Endpoint Protection Manager (SEPM.)
Symptoms:
SEP clients do not check in with the SEPM server but list the SEPM server as the Server in Help and Support > Troubleshooting > Management dialog box.
Cause:
This can result if Internet Information Services (IIS) security settings are set to restrict network traffic. For example, if the Directory Security settings for IIS are configured with IP Address and Domain Name Restrictions to grant access for specific hosts and IP addresses, such as localhost and VPN connections, this issue will occur. The SEPM may reference the server computer by host name, IP address, and also the loopback address (127.0.0.1.)
Solution:
1. Launch the Windows Control Panel and open Administrative Tools.
2. Open the Internet Information Services (IIS) Manager.
3. In the left pane expand <ServerName> (local computer) and expand Web Sites.
4. Right-click Default Web Site and click Properties from the shortcut menu.
5. Click the Directory Security tab.
6. Click the Edit button in the IP address and domain name restrictions.
7. Ensure that local network traffic is enabled for Granted Access.