Username __ is already taken but it isn't

I have several usernames registered under the user panel that are not in use as hosting any domain, yet I cannot select these usernames when in the domain panel. There is a popup button that once worked and showed these users but no more.

My usernames are idle but I cannot implement them to host my domain. If I enter the username in the box I get the “username _ is already taken” message.

How do I apply my established users to a domain?