Why won't Twitter let me change my username?

[Galenos]

I know this is probably considered a question to be asked directly to the Twitter team, but I want to know if anyone has had this problem fixed.
Since yesterday I've been trying to change my username. At first I kept putting in ones that were already taken so I thought up one that probably wouldn't be taken - it wasn't! - then as I tried to log in with the changed name it said that the information was wrong. I kept changing it again and still no such luck. I even tried a different UN. The username, even though it said that it updated, hasn't changed. Even when I search for the account it's under the old name. I want to know why and is it possible to change it?