ARD & Directory Service scanner

Hello, I'm having some problems with ARD (3.3.2) at a client where we are trying to use the Directory Service Scanner in ARD. We've set up the ard_admin group with the right members and also some computer lists. The lists show up in ARD but they are always empty (no clients). OD is 10.6.4 and all clients are 10.6.3+. Has anyone actually gotten this to work? Any hints to send me along the right track? Many thanks in advance.