There are several reasons why your game server may not be appearing in in-game server list.
Firewall Issues
If you can connect to the game itself but it doesn’t show up in lists, check to see if the game has a ‘query port’ or similar and make sure that port is open (and if hosting at home or behind a NAT: forwarded) in addition to any ports used for the game itself.
Too many servers
If the game has a very large number of active servers, some may simply never show up because they never get to the front of the queue.
Latency
Server lists tend to prioritise servers with lower latency to the client. If the latency between you and the server is too high, it may not show up.