So to be honest I am fairly new to value and to theory of light and shade.I noticed that highlights were strong on edges of form, and I was also surprised with how dark the eye in shadow can get.I also noticed that there exist a range of values in the core shadow as well.Overall I already learned a lot doing this, and will likely do it again soon.
Nice job on your shapes. You are headed in the right direction. When you get your shapes worked out well, pay very close attention to the values...even more than you are now...really look and compare. You want to match the values you see as closely as you can. It is important to be very honest about what you are seeing. try to put the accurate value down with each stroke as otherwise you end up having to fix things along the way and being accurate will save you time. Really take the time to observe and compare and choose the right value. If you are off, adjust it, don't keep working and come back to it. You are doing great...just need to focus in on value a little more.
You have done a nice job seeing separate values...and I think that if you watch for the transitions between them, that you will find more gradation than you have currently. Using a soft edged airbrush on skin will be helpful..or one with a more rough edge than a crisp sharp edge. Sharp value transitions denote a major change in planar structure, like how a cube face turns from top to side...usually its pretty severe of a plane change when there is little value transition between each value shape.