If you want to find out more, you can also search the user log.
As administrator, please go to:
Overview => User and Participant Management => Browse System Log
and enter the user name to be searched for.
You can also filter for login attempts by searching via the search "Only Message/Error Level"
The log entry of unsuccessful attempts show why a log-in failed (e.g. "unknown user name").
The user might also have tried his e-mail address or his real name, so it might be advisable to search for this too.