This article outlines the most common problems users have with usernames:
Some usernames may be unavailable for use. To find out if the username you want is available, see Why Can’t I Register Certain Usernames? for more information. If you have a registered trademark that is in use by an account that is potentially inactive or in violation of our trademark policy, please contact us by visiting our Trademark Policies page.
If you're trying to move a username from one of your accounts to another of your accounts, you can do this yourself.
Please note: once you’ve changed your username, it returns to general circulation and may be selected by another account. Also, please note that deactivating an account will not immediately free up the username.
If we haven't properly verified your mobile phone, Twitter may have created an account under a new username with extra digits (like Amy25).