Windows 7 Screensaver not always turning on

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).

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.

What kind of mouse do you have? Some older mice incorrectly register movement on certain surfaces like wood or glass, automatically interrupting the screensaver. In addition, Vista and Windows 7 have a feature where Windows automatically increases the delay if the screensaver is interrupted shortly after activating. If the mouse has indeed been interrupting the screensaver for the whole night, the screensaver might not activate even if you wait for those 15 minutes.

Try unplugging your mouse for one night and see whether the screensaver has activated in the morning.