Debian 9 Stretch: No screen blanking in Gnome 3.22.2

Went to the power management settings in GNOME 3.22.2 to adjust my screen blanking timeout which I currently have set at five minutes, but screen blanking won't kick in after that time, if at all. This also means that my screen isn't being locked either. Any help with this one?

So you tried to adjust the screen blanking in power management and now it no longer works?

That's distressing. What happens when you change it back to what it was before?

Alternatively, you might look at the screensaver settings. They commonly include "blank screen" and "lock screen after" dialogs.

If you're talking about xscreensaver, I thought GNOME didn't install that by default and instead had it's own screensaver based on that. But somehow I didn't get that when I installed GNOME on my system (KDE transplant here). I thought somehow GNOME was able to do the screen blanking without xscreensaver or whatever the other screensaver utility is called.

OK, installed gnome-screensaver, restarted my system, and.... it doesn't appear in my system settings. Is something else missing that I've passed over? If so, can any of you tell me what it is?

if gnome-powermanager is anything like xfce-powermanager, then there's lots of confusing and partly overlapping settings, so please clarify your original statement:

adjust my screen blanking timeout which I currently have set at five minutes, but screen blanking won't kick in after that time, if at all. This also means that my screen isn't being locked either.