It seems like many people have asked how to disable the auto-dim feature (System Settings -> Brightness & Lock -> Dim screen to save power). But I am having the opposite problem. I want this to work, but it often fails, i.e., the screen stays lit after the designated time has elapsed. Sometimes I have been able to fix it by logging out and then back in, but now that doesn't seem to work either. This is on a System76 Ratel Pro with an HDMI monitor. Ubuntu 16.04, kernel 4.4.0-24.
I looked on help.ubuntu.com and found this (again, it was someone wanting to disable the dimming): Terminal command for checking/unchecking "Dim screen to save power"?
So I tried:
gsettings get org.gnome.settings-daemon.plugins.power idle-brightness
Got back 30. So, I tried this:
gsettings set org.gnome.settings-daemon.plugins.power idle-brightness 0
Now it works!