1. Make double click on status bar of this task and check what information is there for each assigned client?
2. Make sure that you have "Inventory Agent" installed on assigned clients.
Open SMP Console ⇒ "Manage" ⇒ "Filters" ⇒ check "Computers with Inventory Plug-in" filter
3. Make sure that you don't have exceeded license node for "Inventory Solution".
Open SMP Console ⇒ "Home" ⇒ "First Time Setup" ⇒ check license state for Inventory Solution.
4. Make sure that same clients are able to execute other tasks, such as "Run script", etc.
5. Make sure that your client computer has Client Task Agent registered with Task Server
6. Make sure that "Altiris Object Host Service" and "Altiris Client Task Data Loader" services are running on Task Server machine.