What you've done so far looks great. But remember, shading is different to shadowing when it comes to drawing. In the real world; yes they're the same concept but what you seem to have done is create more of a casted shadow of the hair strands, which does give the whole piece an element of depth although might not make the head look so much like a 3D round object (I'm doing my best to explain). It looks a little like the hair strands are 'Floating in mid air'. To get the 3D effect of the shading you must always think about: Where the light source is (Where on the canvas it's coming from) and how far away different features are from the camera. Usually you'd have shading away from the light source. The centre bunch of hair has shading on both sides so I presume the 'light source' is directly in front of the drawing. Try connecting various strands together then do close shading to create a more realistic look; which is what you have done in some areas.