After 15 minutes, my display correctly goes to screensaver mode (Photos, Wait: 15min, "On resume, display logon screen"). If I press a key or move the mouse, the screensaver correctly halts and I get the login screen to enter my password.
However, it doesn't always work. Last night, for example, I turned my monitor off (power button), and when I woke up and turned it on, the screensaver hadn't kicked in.
Power Plan is High Performance, monitor-off is "Never", sleep is "Never".
Without changing any settings, my screensaver kicks in sometimes and not others. Overnight it hasn't been turning on lately, but when I went out for dinner it was on when I came back. It's really starting to baffle me.
Update: The screensaver seems to not activate correctly more often when I have updates waiting to be installed (I hate rebooting).
Best Answer
This might be because of some program that you have/had running at the moment you turned off the monitor.
Some programs are programmed on purpose to disable the activation of the screen saver.
For example, a program that does media playback may do this to avoid disruption when watching the movie.