UI doesn't receive EventSystem's calls

Hi,

I have two scenes, both of them have UI with EventSystem.

First is menu scene which pretty much has only button on a plane on a canvas and one script to handle click on the buttons. In this part everything work fine.

The problem is with the second scene. It contains some "game stuff" and has, lets call it, "menu popup". In the menu popup there is a plane with some UI elements. This scene has also EventSystem but it seems doesnt work. No event are called.

When I select EventSystem on the FirstScene in the "preview" window I can see over which of the UI element I have mouse but on the SecondScene the only thing that changes is Position and Delta.

I'm working with unity 5 in the 2d mode.

What I'm missing?