Name lookup directory search working ineficiently

We just switch servers from ver 5 stand alone domino server to a ver 6 notes cluster. Since then the names lookup for some users is no working properly. mainly win2000 users running ver 6.5 client

I have checked all of the following

persons document, address book, ctl-shft-f9 to refresh views, activated exhaustive search, pressing f9 when doing a lookup…etc… but nothing works.

I even refreshed users template and tried diff ones. all he gets when doing a lookup search is actual users or conference room equipment or rooms but no user GROUPS. Groups are showing minimally or not at all. Any Idea?