gnome-control-center display blank has no effect

[John Frankish <j-frankish slb com>]

Using gnome-control-center-3.10.3

Although the gnome-control-center power applet shows:

Display dim when idle <yes>

Display blank <1 minute> (or any other number of minutes)

The display on a laptop dims after about 50 seconds inactivity, but the screen never blanks on either a 
laptop or a desktop.

I do not see any error messages, what would be the best way to troubleshoot the problem?

BTW the display blanks without problems after a set number of minutes using xset or other window managers 
like fluxbox, flwm, etc.

Regards
John