Does this search only look at those members who have made themselves visible on the directory for other members?
Visibility shouldn't matter for a group Owner/Moderator, but there still has to be some profile content. Specifically, if a subscriber has neither an established Display Name, nor a User Name, nor a group profile, you won't be able to find him via a Directory search...for the simple reason that there is nothing in his Directory entry by which to establish a match.
