Do you the same issue on all the clients ?
Check the communication once
if you proper communication between server and client then replace the sylink.
Procedure to Test Connectivity
Open a browser on the client and type the following link
http://server/secars/secars?hello,secars
Where Server is Manager's IP or Manager IP:port (If the iis website is configured to use a different port other than 80 )
If the test is successfull, you should see OK displayed on the browser.
Then you can export sylink.xml from a client group by doing a right click -> Export Communication settings and save the file as sylink.xml
Replace it on the client.
To replace the sylink,
Use Sylink Drop
Or
Follow the below mention steps if SEP is not password Protected.
Start -> Run
SMC -stop
Replace the sylink.xml (Default Location : c:\Program Files\Symantec\Symantec Endpoint Protection)
SMC -Start
Check the client status :)