Shadowing on background

[miles-smiles]

I want to create a group of objects that sit on a colored background where some of the objects are on a grey background and the others are on a blue background. I also want the objects in the group to cast a shadow on that background. I created a background using two rectangles that share an edge and sent it to the back. But when I do that the shadows disappear. I can see the shadows if I set the opacity of the background to 0% which causes the background to disappear. Is there a way to achieve the shadow affect I'm looking for?