XFCE power manager no longer turns off the monitor

[Sam Varshavchik]

At some point recently, my monitor stopped getting turned off after the  
prescribed period of inactivity. I'm using the XFCE power manager, and its  
settings remain untouched.

Manually executing "xset dpms force off" turns the monitor off normally.  
"xset q" shows that everything appears to be in order:

DPMS (Energy Star):
  Standby: 300    Suspend: 360    Off: 420
  DPMS is Enabled
  Monitor is On

So, what else can I check?