When attempting to run a LiveUpdate Administrator (LUA) download schedule, an error is encountered stating "The following updates were found missing on source server Symantec LiveUpdate during download:" followed by a list of missing updates.
Component : LiveUpdate Administrator
The files are getting blocked by a proxy or firewall. Most likely, a list of file extensions (.7z, .m30, .m35, etc...) are getting blocked, causing specific LUA download files to be blocked.
Configure the proxy or firewall to allow traffic to/from the Symantec LiveUpdate servers.
LiveUpdate source server addresses can be found in the LiveUpdate Administrator under Configure>Source Servers. The default http address is liveupdate.symantecliveupdate.com.