Are you sure your xscreensaver is running? You can check the existence of the process with
ps -aux | grep -v ps | grep xscreensaver. If xscreensaver is not running, you should run
xscreensaver --no-splash & disown and you would want to put this command in your .xinitrc or whatever lightdm uses for on login configurations.
If you already have xscreensaver running but the bind does not work, then maybe you are not binding the key correctly. Try following a solution similar to the one in this other question and replace the
Command field with
It is also possible that
lxsession-logout is doing something weird to xscreensaver (I don't know, never used lxsession-logout with xscreensaver), but nonetheless you should execute
lxsession-logout --suspend before the
xscreensaver-command -lock since
lxsession-logout prompts for a logout screen and
xscreensaver-command -lock locks your screen immediately (depending on the fade duration).