Hi there and I hope this will help you........When placing your shadow to your subject try using a darker pencil and start at
the base of your subject hold your pencil closer to the point to make a darker drawing and farther away from the point to make it lighter. Be sure to leave just a faint hint of castlight on your object as it also catches reflected light off the shadow/darker area . I hope this will help!
Layers
I would recommend building up layers gradually, and using a Q-tip, cotton ball, or paper stump to blend between layers. For lighter shadows, use a harder pencil. For darker shadows, use a softer pencil. Hope that helps
Re: Layers
Another thing to consider is pencil pressure. Quick strokes starting from the dark(harder) and going outwards to the lighter(softer) areas to establish a base. Start where it's dark and lift the pencil as you bring the stroke out. Keep doing this to build layers. Also, definitely invest in some tortillons. You can find them at any art or craft store. They are better than blending stumps because they are softer and thus the lead is carried easier over the paper. I swear by tortillons. I used blending stumps for a long time and I will never go back to them. Once you see what can be done with the tortillons, you art will increase ten fold. Hope this helps.