In fact, you can apply an effect to the effect if that will achieve the results you seek. It is wise to link the shadow layer to the object once you have achieve the right look. This is how you can achieve perspective shadows on objects, making them appear to be standing up, or floating in the air. Experiment with the transform tools on a shadow layer and see what you can produce. You may have to play a while, but always keep light in mind.
Place objects near a light source and observe the effects of the shadows.
We have the tool to create any three dimensional effect with effects layers, and you owe it to yourself to learn how to bring the dimension into your work. Work with layers until you can accomplish all basic functions without thinking. Even if you never work on that image again, you can use elements of it by dragging layers or copying effects. Of course, as always, have some fun. There are easier ways to make a living than being a designer.