If the GPO is applying and the installation is failing you'll be getting messages in the event log (Application Event Log on Windows XP, the "Group Policy" log on Windows Vista / 7).
Does Resultant Set of Policy (RSoP) show the software being assigned? If it doesn't check out the "Security" tab of the software assignment itself and make sure the computer has rights to "Read" the software assignment (not the "Security" tab of the GPO-- the assignment within the GPO).
If you're sure the GPO is applying and assigning the software to the PC kick up Group Policy logging (to an insanely verbose level) and sift thru the event log by creating a REG_DWOORD value "RunDiagnosticLoggingGlobal" at "HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Diagnostics" and setting it to "1" on one of the affected computers.
I'm perplexed that you're not seeing anything in the Event Log. That makes me think that the assignment isn't being applied (and that it's a GPO application problem). I suppose there's also the chance that your event log is set to "Overwrite events after x days" and is full, too, and you're not seeing new messages but, hopefully, you've checked that.