Master server list- why do some see server, others not

I recently started a dedicated server for Thinktanks, and have a question.

Why do some players see the server in their list when they go to join a game, and others don't?

Also, in some locations I can see my server, and in other places, I can't.

Why would this happen?

I posted a thread about this on PTT2, but it isn't very clear why this happens.
forums.planetthinktanks2.com/ThinkTanks/viewtopic.php?f=18&t=4552