Change the mode from normal to presentation, and wait for ten minutes, but the monitor still go to sleep. This bug has been confirmed in the arch bug tracker. https://bugs.archlinux.org/task/30743
Changing the "Mode" to "Normal" or "Presentation" does same as running "xset +dpms" or "xset -dpms" in command line, respectively. If some other program changes the setting, xfce4-power-manager does not see it, and you have to choose the presentation mode again. You can check the current state by "xset q". Maybe you are using some screensaver that does not care about that setting? "xset q" tells about some "Screen Saver" setting, too.
This has been fixed in git master and will be part of the next release.