I am running Ubuntu 15.04, but I have had the problem since 14.04. When i start the computer, the login screen shows up but i don't need to enter my password to login. I also cannot lock the computer, the screen goes black until i move the mouse and then I am logged in again.
I tried using sudo passwd [username] and set password that way, but to no avail. Also I have deactivated the automatic login. During my latest try I installed a new greeter (lightdm-gtk-greeter) I do not know much about these but this method also led nowhere.
For some reason on this computer I have a keyring program that I do not have on my Ubuntu laptop.