Can't see other computers on LAN

I have a few computers on my LAN, but the computers are not finding each other. Specifically I have file sharing turned on on my Macbook Air and on my iMac, both running 10.8, yet they don't find each other. The iMac is wired to the network, the Air is wireless. A third computer (Macbook, 10.7, wired) sees the iMac, but not the Air. A 4th computer (also Macbook, 10.7, wireless) sees neither.

The computers are all connected to a Verizon FIOS router/modem.

Any suggestions as to what might be the problem or what to look for?

Thanks!

Dan