cannot see other machines on network

Here's my confusion:
There are 4 machines running on a Routed network using TCP/IP. Machine1,2,and 3 are all using 98SE. Machine 4 is using ME (yuck ). They are all members of the same workgroup but cannot see each other in through Network Neighborhood. They used to see each other fine but now the only way I can get them to see each other is by installing NetBui protocol. Why? They also all have consecutive IP addresses and same subnet mask.

 

What are all the PC's connected through....a hub/switch and the speed of that device. What is the router config and how does it figure into your setup?