Here's what I do.
On those machines that are a problem, I first download the full Superdat installer and run it manually on each machine. It installs the latest scan engine and DATs on the machine. This can frequently fix the issue and allow the computer to start automatically updating correctly again.. See the link below for that download site. If you haven't been to the site recently, you'll need to click on the "I Agree" button before the site will show you the "sdat5402.exe" file.
http://www.mcafee.com/apps/downloads/security_updates/superdat.asp?region=us&segment=enterpriseIf that doesn't fix the problem, then there was an old issue which caused the machine to be unable to update. The "FrameworkManifest.xml" and "sitelist.xml" files which reside in the C:\Documents and Settings\All Users\Application Data\McAfee\Common Framework" directory can become corrupted.. Copying both of those files from a known "good" computer and replacing the old, bad copies on the problem machine will frequently fix the update issue.
Hope this helps.
Grif