3 tips:
1. Make sure your light and shadow casting objects are on the top of the layer OR on a different higher layer. A while back I was making a game using shadow casting and this seemed to do the trick.
2. Make sure you are standing at an angle that a shadow would be drawn. Make sure the light has some space behind the shadow object. In other words, is it possible for it to draw a shadow in this case
3. If these dont work try setting your shadow color to a different color for testing purposes.