Start with using the Remote Altiris Agent Diagnostics (RAAD) tool to look at the Altiris log on the client:
http://www.symantec.com/docs/HOWTO21449
You might need to turn on trace logging on a sample client:
http://www.symantec.com/docs/HOWTO1461
[HKEY_LOCAL_MACHINE\SOFTWARE\Altiris\eXpress\Event Logging\LogFile]
"Severity"=dword:0000000f