Hey there! Nice illustration so far! :D So, you need to add some shadows, even if they’re just flat shadows, so they can help define the volume of the forms. They should also be relatively strong shadows, because if you shade using very soft values, they won’t do much to suggest volume. As a recommendation, try to avoid front lighting, since it usually flattens the forms and kills the sense of volume. Because of that, it’s generally better to use 3/4 lighting (whether it’s from above, below, or at mid-level). And if you want light coming from behind, you can use artificial lights to help emphasize the volume of the forms. Another option, if your goal isn’t necessarily to shade, is to suggest volume using linework. You can draw contour lines that help indicate the form—for example, along plane changes (where the terminator would be on organic forms), or even through the folds of clothing. Line quality can also help: use thicker lines in shadow areas, in ambient occlusion, or where forms overlap. Hope this helps!
