First try by replacing sylink file in one client and see.For this
export the communications settings from the server
In the console, click Clients.
Under View Clients, select the group in which you want the client to appear.
Right-click the group, and then click Export Communication Settings.
In the Export Communication Settings for group name dialog box, click Browse.
In the Select Export File dialog box, locate the folder to where you want to export the .xml file, and then click OK.
In the Export Communication Settings for group name dialog box, select one of the following options:
To apply the policies from the group from which the computer is a member, click Computer Mode.
To apply the policies from the group from which the user is a member, click User Mode.
Click Export.
If the file name already exists, click OK to overwrite it or Cancel to save the file with a new file name.
To finish the conversion, you or a user must import the communications setting on the client computer.
For this
Copy the SylinkDrop.exe file to the desktop of the client you want to be managed.
This tool can be found on CD 2 of the Symantec Endpoint Protection 11 installation files, in the folder "\TOOLS\NOSUPPORT\SYLINKDROP". It can also be obtained from Symantec Technical Support.
Double-click SylinkDrop.exe.
When prompted to select a Sylink.xml file, select the Sylink.xml you created in previous steps.
Click on update sylink.
After replacing it will give a message as sylink has been replaced successfully.
Or
You can use this method also for changing the sylink file
On the client computer , click Start > Run, type smc -stop, and click OK.
Copy the Sylink.xml into the C:\Program Files\Symantec\Symantec Endpoint Protection folder, and replace any existing Sylink.xml file.(This name is important)
Click Start > Run, type smc -start, and click OK.