Missing computers in network places

Using Windows 2000/xp (no domains) when I browse the network places for other computers in my workgroup, no computers show up? Sometimes disconnecting the network cable and replugging it in will allow me to again see the other workgroup computers but after a period of time they will disappear again? The connection seems ok and we can ping the other computers on the network, we can also see a display of the workgroup name but no computers are diplayed as being in the workgroup? Problem just started a week or so ago?