I thing you will need to chnage the sylink file on that client
Can you follow this
--> To copy the Sylink.xml manually to the SEP client
1. On the system that has the new Symantec Endpoint Protection Manager, browse to C:\Program Files\Symantec\Symantec Endpoint Protection Manager\data\outbox\agent\.
2. In this location, there are two or more folders with 32-character alphanumeric names.
3. Each folder represents a group in your Symantec Endpoint Protection Manager site. Inside of each folder is a file named LSProfile.xml.
4. Open the LSProfile.xml file to view which group the alpha numeric folder belongs to find the XML tag: <GroupInfo Name="X" where X is the name of the group.
(Do not edit LSProfile.xml. Any edits to the LSProfile.xml file is not recommended or supported, and can cause serious communication problems.)
5. Once you have located the correct group folder that you would like the client to belong to make a copy of the Sylink.xml file and move it to the desktop of the client that you want to be managed.
6. Click Start
7. Click Run
8. Type: smc -stop ( this will stop the Symantec Management Client service)
9. Navigate to: C:\Program Files\Symantec\Symantec Endpoint Protection on the SEP client
10. Replace the new Sylink.xml with the original one in the C:\Program Files\Symantec\Symantec Endpoint Protection folder
11. Click Start
12. Click Run
13. Type: smc -start to start the service
14. Open the Symantec Endpoint Protection Manager
15. Click on Clients page
16. Highlight the group
17. You should see the client in the group of your choice
Note: ( if the client is in a different group just right click on the client and choose Move, and choose the group you want the client to be moved to)
Please mark this as solution if this works