Just to double check, you are, in fact, on a LAN with everyone who wants to play, right? ie: everyone who wants to play is physically in the same location, physically connected to the same router/switch/hub, and not each in their own homes?
If requesting servers fails to work, you can always join games manually, ie: the host checks his/her IP, and say, it shows as 192.168.0.100; everyone can then connect directly with that IP (and their IP's would be 192.168.0.101, 192.168.0.102, etc)
Ensure that whoever is hosting knows what port they are hosting on, and everyone else needs to type that port in, both to connect, and ALSO to have the server show up in the server list.