Hello,
What version of SEP 11.0 are you carrying?
Is this issue is happening only on 1 machine? or few other machines?
What OS is this machine running on?
Did you repair the SEP client?
Could you check the version of Liveupdate in the Add/Remove Programs?
Did you follow this Document : Symantec Endpoint Protection: LiveUpdate Troubleshooting Flowchart
http://www.symantec.com/docs/TECH95790
If nothing is working, please upload the log.liveupdate from the Client machine.
Hope that helps!!