List of that computer which is not be report to altiris server from last 30 days to verify the reason. Please advice to capture it?
Inbuilt report availble to find it.
Check the thread
https://www-secure.symantec.com/connect/forums/altiris-report-type
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Reports->All reports->Discovery and Inventory->Inventory->Cross-platform->Inventory Agent->Inventory Diagnostics->Under you can find (Computers that have not reported inventory in last N days)