Ubuntu – Kubuntu 15.04 doesn’t suspend when closing lid

I recently installed Kubuntu 15.04 64bit on my laptop, precisely an Asus UX32VD (it has got 2 graphic cards, Nvidia geforce 620m and Intel).
Everythings works fine as it was with Kubuntu 14.10, but now I have a great issue, that is when I close the lid nothing happens. As I set from the kde power manager when I close the lid my laptop should be suspended but this doesn't happen, so the laptop screen remains on.
I searched for a long a solution on internet, but nothing allowed me to solve the problem, so I was forced to open this ask on askubuntu.com. For example I try to:
1. install nvidia restricted driver using ppa: xorg-edgers (preciselly nvidia-352 version);
2. install pm-utils and laptop-mode-tools
This is my xorg configuration if it returns useful for you:

Section "ServerLayout"
    Identifier "layout"
    Screen 0 "nvidia"
    Inactive "intel"
EndSection

Section "Device"
    Identifier "intel"
    Driver "intel"
    BusID "PCI:0@0:2:0"
    Option "AccelMethod" "SNA"
EndSection

Section "Screen"
    Identifier "intel"
    Device "intel"
EndSection

Section "Device"
    Identifier "nvidia"
    Driver "nvidia"
    BusID "PCI:1@0:0:0"
    Option "ConstrainCursor" "off"
EndSection

Section "Screen"
    Identifier "nvidia"
    Device "nvidia"
    Option "AllowEmptyInitialConfiguration" "on"
    Option "IgnoreDisplayDevices" "CRT"
EndSection

Finally I would point out that If I try to manually suspend the system, it works.
I hope some of you can help me please, also because I'm scared to leave my laptop in bag while it is turned on again. Thank you in advance