I'm on trusty and I can't lock my computer by any means. When I press Super+L or use the Lock button in the panel, the behavior is the same: the screen turns black and if I move the mouse, my desktop pops up again, without ever locking.
I'm on Trusty x86 and I confirmed that if I create a new user account, I can lock the screen in that account. I checked the Lock settings in "Brightness & Lock" and everything is grayed out, as the following screen capture shows. How can I enable the usual system lock again?