I have had the same issue since open beta.
I think once the servers get so full, the data centre doesn't show up on the list. It is generally rectified by resetting the client and logging back in a few minutes later. Problem is, once you get back in, even if you can see your character doesn't mean you will be able to login to the game. Ultros certainly has been unavailable (without a queue) for the last hour at least.