Helpdesk's Active Directory import fails to sync new users or computers.
The users or computers do not exist in the Notification Server and therefore there is nothing for Helpdesk to sync from.
Helpdesk does not access Active Directory (AD), it only syncs from the Notification Server's (NS) Resources list (found at Resources > Configuration Items/Organizational Types, or if CMDB Solution is not installed at Resources > Organizational Structures), regardless of how the users and computers there got there, whether that was from an AD import, etc. The NS accesses and manages the Microsoft Active Directory Import (found at Configuration > Server Settings > Notification Server Infrastructure > Microsoft Active Directory Import). Until this NS process is configured and working, Helpdesk has no ability to sync new users and computers that came from AD into the NS's Resources list.
Depending on how often the Microsoft Active Directory Import has been scheduled to occur in the NS, Helpdesk may not see new users or computers (that originated in AD) for days, weeks or months. For example: If AD is set to only import new users once a week, it may take as much as one week plus one hour to see the user appear as a Contact in Helpdesk. This factors in if the user was just added right after the last AD import, so that a maximum of one week is needed before the next import occurs, and then one hour maximum for the next Helpdesk sync from NS, if the last sync just occurred before the user was added to the NS Resources list.