creating 2.5D Game - shadow on invisble 3d mesh covered by 2D sprite

Hi,

I am trying to create 2.5D game using Adventure Creator plugin.
I am using 2D background, 3D character, and 3D room objects.
the mesh of the 3D objects are used for collisions and shadows only, so actually they are invisble.
i am also using 2D sprites for objects in order to make the character look like he is behind them (images cropped from the 2D background itself)

My problem is that the shadow reflected on 3D object overridden by its 2D sprite