I have sys admin profile. I dont understand why I am not able to log as some users which have the same custom profile. For example in the image, "Analyst T.O" has the same profile as "Analyst, Credit" and "Dorado William" but I the link "Login" does not exit.
Well, finally I found the answer.
The reason for which "login" link is not available in some users, is because those users have not granted access permmission to the admin.
Those users must go to: My settings/Grant Account Login Access and select Access duration for Your Company's Administrator.
After that you will be able to see "Login as " link: