I found that if
xfce4-power-manager does not work if it is started with
/usr/bin/xfce4-power-manager, the process is there but no power icon would be shown, the Power Manager in Settings does not work either.
However if I start xfce4-power-manager with command
xfce4-power-manager --no-daemon option, everything works normally. Is there anyone facing the same problem?