Hey there! Beautiful illustration so far! The problem with not being able to give the hair volume is that you're not thinking about its organic shape. In my opinion, the best way to think about its organic shape is to think of the hair as ribbons. Thinking of the hair as ribbons makes it easier to determine which areas will be exposed to light and which areas will be in shadow. At the end of the day, it's all about keeping the organic shape in mind so you can separate light and shadow more accurately. Hope this helps!