Did you check out KB 1328? Where does that get you?
Do some clients allow remote control, and not others -- or are you unable to remote any client at all?
What status icon do clients show -- greyed out, on but no user, on and logged on user, or a mix?
What are all the error messages you get when you try to remote control a live computer?
Is the Windows firewall correctly configured to allow exceptions to the DS server? How do you know the client can see the server?
Because it's all clients, it's likely the problem is server-based. What happens when you check port 402 according to KB 1328?