As its handled by policies, plugin installations usually only show up in the normal agent log. Package download, execution etc.
To the problem itself, the Symantec Management Agent on the affected PCs is up to date? Which version? Is the Symantec Management Agent Upgrade Policy (Non Site Server) turned on? With MP the Agents should be version 8400.
If they are not up to date they dont get any plugin installs if I remember the target right.