mclemson,
It seems, that some other update was installed and made APSB11-03 not Vulnerable for this client - maybe it was some superseding update. Therefore, Update Advertisement disappeared from the client before it got executed. To troubleshoot this situation, please check InstallLog.csv file, which resides in the client's Patch Agent directory: C:\Program Files\Altiris\Altiris Agent\Agents\PatchManagement\InstallLog.csv.
Also, please check Compliance by Computer - is this update still in the Vulnerable List for this computer?
If it is Vulnerable, please check Update Advertisement - is it actually enabled in Altiris Console?
Then, if Update is Vulnerable and Advertisement is Enabled - it should be registered in Software Update Tab of Altiris Agent and therefore be installed via AexPatchUtil.exe /Xa.
The other option, is Update is not Vulnerable on the client but You believe it should be. Then the problem can be connected with Inventory Rules.