I'm assuming
this is the interview we're talking about? I'm not sure what more he could have done there since he basically got him to admit he authorized the leak.
I'm sure that Russert missed opportunities to call the administration out on things, anyone in his position would have. I don't read political blogs, so I probably have a different perspective than you, but what I saw every week was someone that did as much research as they could and always tried to make people on both sides accountable for relevant issues.