Issue: Users are not indexed in the database that should be.
Cause: User replication issue.
In Lync Server 2013, Lync Server User Replicator will pick up the changes from Active Directory and update the Lync Server user database based on a configured interval. Lync Server User Replicator will also propagate the changes to the RTCab database quickly without the administrator having to run Update-CSAddressBook. Administrators will only need to run Update -CSAddressBook if the Address Book file download is enabled.
You need to have RTCUniversalUserAdmins, RTCUniversalServerAdmins permission or equal permission to run this command.
Run Update-CsAddressBook to synchronize all accounts.
PS C:\> Update-CsAddressBook