Seeing my WPMU sites in the MY Sites dropdown menu.

I have three WPMU networks, each with multiple active sites.
In one,, I can "see" my entire site listing in the drop-down menu in the network management panel.
I also have the MultiPost plugin installed, and it "sees" all of my sites.
In my two other networks, however, neither of these things is true.
I see - and Multipost sees - only my main network site, and none of the others.
Can you help me to resolve this?