Use the Remote Altiris Agent Diagnostics (RAAD) tool to look at good and bad agents, there's an Inventory tab that will tell you if it's being collected:
http://www.symantec.com/docs/HOWTO21449
You could also try creating Managed Delivery Policies to collect inventories, there's a KB article on that but I can't find it. Edit: Found it all, there's a Tech article that now contains a suggestion:
http://www.symantec.com/docs/TECH187623
And there's a blog posting:
http://www.symantec.com/connect/node/2431411
There's a Hotfix 1 for 7.1 SP2 MP 1.1 that contains a revised Inventory agent:
http://www.symantec.com/docs/HOWTO81832