Are you playing on PXO or with virtual LAN? What FSO build and mods are you using?
For PXO, if you are using FS1 or FS2 co-op then there should always be at least one server shown (the PXO ones). If you just can't see your own server then it's likely a firewall issue. Newer FSO builds, such as one of the 24.2.0-RC ones, support UDP hole punch and can generally get through firewalls without any special setup on your end.
For virtual LAN, the game uses mDNS to find local games and this can sometimes flake out a bit. The best fix is to get the virtual LAN ip address of the game host and enter that along with the port number in the in-game options screen on the clients. The default port is 7808. So if the ip address of the host is 192.168.10.20 for example, then on the clients go to options, click the multi tab, and then Add in the IP address box. Type 192.168.10.20:7808 and press enter to save it. Then when the client goes to the server list they should see the game available.