Have a look at your sylink.xml file on the client.
It might be corrupted or the SEP server ip is wrong.
If the sylink.xml is correct you can manually import it to a client to see if it then gets a green dot.
To convert an unmanaged client to a managed client
1 In the client, click Help and Support, and then click Troubleshooting.
2 In the Management dialog box, under Communication Settings, click Import.
3 In the Import Communication Settings dialog box, locate the sylink.xml file, and then click Open.
4 Click Close.
I could not find any download for SEP_SupportTool.exe
I got the tool from Symantec when I had started a support case.