Shared computer in finder window

[old-wiz]

I have two active systems, an iBook and an iMac 24", both running the same version of Leopard. With both systems up and connected to the LAN, the finder window on the iBook shows the iMac as a shared computer and can connect easily to the iMac and move files back and forth. However, on the iMac, the iBook does not show up in shared computers; I can't figure out why. Both systems have file sharing on, both have finder preferences set to show shared computers. Any thoughts? This used to work, but I don't remember when it started to fail - perhaps when I updated the iBook to Leopard? Perhaps a software update?