Event Trigger is not firing (Unity 5)

I made myself an Inventory systmem, that is quite similiar to the one used in Terraria or Minecraft. I used an Event Trigger on each of the Slots, that basically changed the color when the mouse hovers over it or exchange the item in the Slot for the Item in the "Mouseslot"again just like Terraria (I used Pointer Enter, Pointer Exit and Pointer Click). In a Scene where there was only the Canvas and the UI Images in the scene, it worked perfectly,but when I tried to move the whole Cancas as a Prefab (Canvas is set to screen space - overlay) it did not register the mouse anymore. There is an Event system in the Scene, the graphic raycaster settings were the same I used in the other scene... My main Scene used an FPS Controller from the standard assets. So I tried it in another scene with the exact same FPS Controller and it worked again... There is only one Canvas in my main Scene and nothing blocking it... Im kinda clueless