Screen won't enter power down - 12.04

Very annoying problem here with Ubuntu 12.04.

I have set up the power saving to turn the screen off after 10 mins of inactivity. Instead, the screen simply goes black but the backlight remains on. I have also tried running "xset dpms force off", "xset dpms force standby" and "xset dpms force suspend" - all of which have the same effect.

I'm running a Dell Latitude E6410 with the non-free nVidia drivers on the Unity desktop. This particular feature works on other distributions.

I have tried the following:

- removed gnome-screensaver (no effect)
- set X screen parameters manually, via xset and xorg.conf (no effect)
- xset force commands (no effect)

Am unsure of where to proceed - this is extremely annoying.