From here:
gnome-not-locking-screen-automatically

We have some problems with LightDM / LightLocker running under Gnome-Shell.

With GDM all is working right out of the box.

But with a new installed Antergos and LightDM session locking with Keystroke [SUPER+l] does not work, seems to be a problem on LightDM caused by Gnome-Shell do not give a response to it, or the wrong one…

I read that if GDM is not running Gnome-Shell give the response to gnome-screensaver…
LightLocker is build out of gnome-screensaver but seem to be outdated or failed in the combination.

We need a working solution for Gnome-Shell standard installation without replacing LightDM with GDM by hand.

For me in a way put a light-locker starter inside /etc/lightdm/Xsession like this:

/usr/bin/light-locker &
echo "X session wrapper complete, running session $@"

exec $@

locking the session works with [SUPER+l] keystroke… but not all the time… i can not reproduce when or when not… but mostly it locks the session and i have to put password to relogin the session…