Lock screen Ubuntu 16.04

I am unable to lock my screen either using CTRL-ALT-L or waiting for 5 minutes (inactive time option). After pressing the combination of keys (or waiting the 5 minutes), the screen turns off but it does not ask for any password to return to the desktop. Can anyone help me please? Thank you!

Here are my settings:

I solved my problem. There was an option in the Unity Tweak Tool that was preventing any screen lock. Desktop lock checkbox must be disabled. Thanks to https://askubuntu.com/a/91719/219893

Enable the "LOCK" switch in the settings "Brightness and Lock" dialog.

I see that the "LOCK" switch is disabled on your screenshot.

Then it would ask for a password after blanking screen or pressing CRTL-ALT-L.

To also ask for password after standyby: Then you have to ensure that the checkbox "require my password when waking from suspend" is selected.

I have attached a screen shot of my dialog here: