First Check Outlook Web Access as this will always be up to date. If the staff members are missing from OWA then this is a different problem.
If the staff members are in OWA try a manual download
Outlook > Tools > Send/Receive > Download Address Book
Uncheck Download changes since last Send/Receive > OK
If this fails then repair the Outlook Profile:
Outlook > Account Settings > repair > Next
If the wizard reports that Outlook need to be restarted then restart Outlook and retry the manual download above
If the manual download fails start Services.msc (elevated/run as end user is logged in)
* Automatic Updates
* Background Intelligent transfer Service
are both set to Automatic and are started.
If not, make those changes.
Return to Outlook and try the manual download again.