Try deleting these clients from SEPM console then on any of the client right click on the SEP icon and Update Policy.
If doesn't help then
Do you have Proxy on these clients ? Try this
Back up registry
1. Click Start, and then click Run.
2. In the Open box, type regedt32, and then click OK.
3. Locate HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Connections.
4. Right Click on Connections from the menu, click Export.
5. In the Save inbox, select a location in which to save the .reg file, type a file name in the File name box, and then click Save.
Remove DefaultConnectionSettings & SavedLegacySettings
1. Delete the following registry keys:
HKEY_USER\.DEFAULT\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Connections\DefaultConnectionSettings
HKEY_USER\.DEFAULT\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Connections\SavedLegacySettings
2. Reboot the system.