Ensure the Symantec Management Agent and the Monitor Agent are up to date on the problem computer.
In the console go to Settings>All Settings>Agents/Plug-ins>Monitoring and Alerting>Monitor>Windows>Monitor Plug-in for Windows x64 - Upgrade (or whichever upgrade policy is appropriate to the OS) then make sure that the policy is enabled.
Click on the target in the Applied to section, click the pencil (Edit) button, click Update results to update the target, then make sure that the problem computer is not listed as a computer that needs to be upgraded, otherwise wait for the upgrade to run.
Make sure that Monitor services and processes are running.
Open Task Manager on the problem computer, go to the Processes tab, and verify that AexMetricProv is running (or Monitor Metric Provider on Site Servers).
Go to the Services tab and make sure that MetricProvider is running.
Open Services and make sure Altiris Monitor Agent service is running.
If none of these services or processes are present then the Monitor Plug-in was not installed completely.
Uninstall the monitor plug-in with the appropriate uninstall policy under Settings>Agents/Plug-ins>Monitoring and Alterting>Monitor or from Site Server Settings if it is a site server.
Reinstall the monitor plug-in or monitor service.
To accelerate this process click Send Basic Inventory from the Agent and run Delta Resource Membership Update from the console between every step.
In the console go to Settings>Agents/Plug-ins>Monitoring and Alterting>Windows>Configuration>All Windows Servers (or the settings page appropriate to the OS) then set View: to Targets and make sure that the problem computer is listed and that the policy is turned on (this will not include servers with Monitor Service).
Refresh the policy file on the problem computer.
For versions 7.5 and above: go to C:\Program Files\Altiris\Altiris Agent\Client Policies\ and delete all the files. Then open the Symantec Management Agent and click Update Configuration to recreate the policy file.
For versions 7.1 and below: On the problem computer go to "C:\Program Files\Altiris\Altiris Agent\<x64 or possibly x86>\Monitor Agent" and check the file Config.xml (depending on the OS of the monitored server, it might be at C:\Program Files\Altiris\Altiris Agent\Monitor Agent). If it's smaller than 1 KB stop the AexMetricProv process, delete Config.xml and the RuleState file and then restart the service.
Open the XML file and confirm that the computer received monitor policies. Use control+f to search for the name of a monitor policy or search for "metric".
If the monitor policies are not listed review the the policy configuration and make sure they target the computer.
If the policies are listed and all the previous steps are in order the agent may be corrupted. Perform a clean uninstall of the agent with the following commands and then reinstall it: