The ranking system in roles works from the highest value being zero (0) if a role is set with a number higher than this, this means it's a lower ranking in terms of the user's ability to sign in as another user. Which is why the sign in option is not visible on some users found in the users' page.
Click on the Admin Cogs
icon at top of page Click Organisation Administration, then Roles.
Edit the role required and enter the Ranking Number within the details tab. (the highest rank is 0).
Click the Save Settings button.