You perform an AD Sync on an Active Directory Organizational Unit (OU) containing computer objects; and the OU is not listed in the domain.com PDQ Inventory Tree item.
NOTE: By default, PDQ Inventory does not list AD OUs that do not contain computer objects.
PDQ Inventory only considers and enumerates objects as existing within an OU if they have the dNSHostName attribute set. For example, if a computer or set of computers have been pre-staged in the OU, those objects will not have the dNSHostName attribute.
You can manually check whether a computer has this attribute set by opening Active Directory Users and Computers (ADUC) and performing the following:
You can also check this using PowerShell; here is an example:
Get-ADComputer -searchbase "OU=Accounting,OU=Finance,OU=HQ,DC=domain,DC=tld" -SearchScope 2 -filter 'ObjectClass -eq "Computer"' | Select -Expand DNSHostName
If no computers are listed when the PowerShell command is run, then no machine in that OU has the dNSHostName attribute set.
Add the machines to the domain and the issue should be resolved.