Your ans for :- Some hosts cannot connect to the main antivirus server.
First stop SMC Service and then Go to C:\Program Files\Symantec\Symantec Endpoint Protection path and replace SyLink.xml file from another system which is reported to server.
Some hosts show its connectivity but they cannot make protection from the different types of threats.
You can try this :- Start your system in safe mode and scane your system. i hape this mathod is helpfull.
Some time server sends notification that is "File system Auto-protect is not function correctly. Your protection definitions may be damaged or your product installation may be corrupt."
Please replace SyLink.xml file. if it is not work then reinstall symantec in client.
Device or folder scanning process does not perform.
may be you disable the schedule scaning.