Assuming you are trying to display System.User:
This is likely caused by the way you have set up your user roles, specifically which (other) roles a specific user role is allowed to manage. I don't know the exact specifics of how this is set up, but I believe that you only are able to read attributes of System.User objects which have a user role which you are allowed to manage based on your own role(s).
(So say you have the role User, and this role is not allowed to manage users with the Administrator role, you would not be able to see the username for objects created by users with the Administrator role.)