It depends on the client's IP (who opens the serverbrowser) if he sees your server or not.
Pretty shit system, but that's it...
Your server is listed in the masterserver, but with a low rank (the order which the masterserver gives the gameserver IPs).
There is a very interesting piece of paper out in which someone presents a very good idea how to improve this system:
http://simula.no/~griff/nossdav2008/03-08.pdf
I've already sent this to Alfred months ago and he surprisingly responded to it. But I think it has very low prio hat valve. Hope never dies...
cheers
Marcel