It's because I disabled the screensaver but I didn't remove it from the autostart program list. So the screensaver somewhat block the power-manager to shut down the screen.

I am running xfce 4.10 on my laptop. After starting the xfce, the xfce power manager can't control the display by defaul, for example, reducing the brightness, switching off the monitor if the laptop is inactive for the time that I set in the preference.
However there are some ways to make it work:
1. quit xfce4-power-manager and restart it again, it will work well
2. run "session and startup" manager, go to tab "session", and change power manager restart style to never or immediately, then when I log in next session, it will work well. But then it stops working again
3. if I run xfce4 as root, power management will run it perfectly, smoothly.
4. open the xfce4 power manager and change the time and close. It works again
5. The second is to unplug in and replug in the laptop the ac adaptor. The function comes back too.

So, I want to know what I could do could make the power manager work properly?

I am running xfce 4.10 in Gentoo

Thanks