Well, you've got some fairly heavy shadows in there, but there are a few shapes that could do with more definition. Considering the direction of the light, and the actual shape of a human face, I did a quick draw-over to show you where you could push the shadows a bit more (I'd upload it straight to the forums, but Tapastic is being grumpy and not letting me do that today).
You've got the solid shadows beneath the chin and over the eyes, but there's a lot of shapes you're leaving out. Considering the direction of the light, there should be a fairly heavy shadow on the underside of the nose, for example, and the top lip would fall in shadow as well. Right now, they've only got a kind of grey blush to them, and considering your use of sharp shadows around the eyes/cheekbones, it ends up looking a bit flat. Those are 3-dimensional shapes, and need some definition.
Also, you could probably push some darker shadows within the shadow over the eyes; that's not just a flat area of the face either. There are brow-ridges and eyelids and the bridge of the nose and the forehead and everything; parts of them should fall into deeper shadow than others.
Oh, and consider that the strands of hair are also blocking the light, and should result in shadows on the face as well.
All in all, it helps to really consider the 3-dimensional shape of the face in the sketch-stage, and know that you aren't just plopping areas of grey onto a flat surface; there's a whole bunch of complex shapes in there!