hi there..
I created a Root UI Object, that contains ALL the GUI of my game as children. To switch UIs, i use SetActive(true/false) to their WindowBG Object. So my Hierarchy looks like this:

Now, i created a PopUop Infowindow.

This should stay in the foreground when activated. And all Buttons or GUI in the Background should be blocked. Therefor i added a background Blocker. This is just a simple Sprite with a black background made with alpha set to 50 (of 255). So the background gets a little bit darker.
The Sprite has a Box collider attached, to cover the whole area.
To make the Sprite been set on a higher level than all the other UIs, i set its depth to 900. The Infowindow has a depth of 1000.

Now, when i SetActive the InfoWindow, the Window itself activates the Background Blocker.
BUT - all other Buttons can be highlighted and pushed again.. as if the collider isn´t there..

What could be wrong.. As if i remember right, NGUI uses thr depth value for the collider system..
And for testing (as you can see on my inspector Screen), i pushed the Background Blocker´s Z position in front of all others (expect infoWindow)
Means:
Camera > InfoWindow > Blocker > Others
any ideas?