Missing users in People Search

Hi All, I"m using RT 3.8.4 that’s connected to Active Directory for LDAP
lookups. For the most part it works fine, except there are a subset of
users that my installation can’t find. I can’t find any discernable
difference in their entries vs. ones that work. Any tips on how I might
further troubleshoot that? The ones I’m having trouble with don’t log in
directly, but I should be able to assign them a watcher role in the People
part of the ticket. I’m not getting any logging from that operation. For
example, there might be 10 people named “Smith” in the company, but when I
search for “smith” I only get 8 of them.

Any tips or pointers appreciated. Thanks!