Airport Utility not seeing Base Station or Express, but others do

I have a newish iMac with 3.06 GHz Core i3 connected to an Airport Extreme and a printed connected to an Airport Express. I have Airport Utility 5.5.3 (the latest). I'm also running 2 older iMac G5s on the same network, and one of those is connected directly via ethernet to the Airport Extreme. Every now and then the printer refuses to work and says it's offline. Usually I can overcome this by restarting power cycling the printer and restarting the Express via Airport Utility. A few times (like now) when I open Airport Utility it's unable to find any Airports (even if I hit rescan multiple times), preventing me from restarting the Express, even though it clearly shows that I'm on my usual network. AND both my older iMacs show the Airports in the utility. What's going on?