User information is scanned when a user is found to be logged on. If the user is not logged on or if the computer is not being scanned due to firewall or other issues, the user information will not be returned.
You could also try refreshing user details through the configuration console. The "Refresh active directory user details" option can be found under Scanning Server/Your Server/Options. Restart the Lansweeper service if you immediately wish to perform the refresh action.