If the 'Lock screen when system is going for sleep' option is turned on in the power manager settings, xfce4-power-manager doesn't suspend the computer when the 'When laptop lid is closed' is set to 'Suspend', and I close the laptop lid. The 'Suspend' button in xfce4-session-logout works correctly: it locks the screen, then suspends the computer. If the 'Lock screen when system is going for sleep' is turned off, the power manager works well: if the lid is closed, it suspends the computer.