There are many possibilities. The three most common:
1. You have a hardware or software firewall that is keeping your server from being seen.
2. Everything is working, your server is out in the Internet, but the current bug in Raven's Master Server List is keeping your and 90% of the other servers out there from being listed in the ingame browser. Join the club. It would be visible in the All Seeing Eye http://www.udpsoft.com/eye2/index.html
3. Your server is getting out, it is
listed in the ingame browser (for now at least) but no one is choosing to go to your server. There are almost 500 servers running at any time, so you have lots of competition for players.