Here's my scenario. I have a mini, iMac and Air networked. All now running Mountain Lion.
When any of these computers go to sleep, they become invisible to the network. I can't see them or "wake" them even with the "network wake" option enabled in power settings.
So, if I want to do a quick ransfer of files from my Air to my iMac -- no go, if the iMac is sleeping. If I get up, go to the room with the iMac, press it's keyboard to wake it, it then shows up on my Air's finder window side bar, and I can exchange files.
Why is Mountain Lion not waking the networked computer when I try to connect to it?