The Symantec Management Agent on a client PC runs in the context of the Local Built-in System account, this has no access to network shares. I don't like installing across a network so i always create a Package in the console to go with the command line.
Otherwise you can use the Advanced options of the Task or Policy to have it run using a domain account that has rights to \\server\share.
One gotcha I found was that you always saw the Office 2010 splash screen even with all silent options selected unless the config.xml was in the root of the package with setup.exe.