Download and run the SymDiag tool on an affected client to see what it shows. If content is corrupt, it will give a "Fix" option.
You can also try installing the Intelligent Updater file on the affected client:
http://www.symantec.com/docs/TECH102606
These may be of help:
http://www.symantec.com/docs/TECH95790
https://support.symantec.com/en_US/endpoint-protection/troubleshoot-liveupdate.54619.html