For values keep in mind: 1. The closer the surface is to the source of light the brighter it becomes while closest shadows become darker. While there is highlight in her forehead, her hair is nearly flat due to lack of depth in values. There is a strong shadow under her lip but the lip itself is too dark. 2. Try to avoid to draw with pure white and pitch black - it will leave you no room for extreme highlights like on nose tip and will potentially make drawing look flat, too. 3. Light reflects back off surfaces and potentially lightens surfaces that otherwise would be in dark shadows (see plane above her eyes). 4. Skin lets part of the light through making some parts lighter, e.g. ears and bottom part of the nose. 5. When adding a shadow, consider why it is there (in your case the shadow underneath her nose was too big for the shape of the nose). When asking for feedback on such studies, try to add reference as well - it makes it easier to see where the issue is. I am a student (Rei on DC) so take the feedback with a grain of salt.
