We had a major issue where Syman­tec Anti-Virus would not update some (but not all) 64 bit clients. To make a long story short, it ended up being cor­rupt files that had pre­vi­ously been down­loaded by Live Update, and lived in the Live­Up­date direc­tory on the server. To clear up the prob­lem we had to…

In Live­Up­date Admin­is­tra­tion Util­ity ( v1.5.7.19 )

Tools — select “Clean Up Down­load Direc­tory.“
Options — select “Retrieve new and pre­vi­ously down­loaded updates.”

Go into your Live­Up­date folder and make sure all the files are gone (delete any that are still there.)

Back in the Live­Up­date Admin­is­tra­tion Util­ity click “Retrieve” on the top right corner.