Hello there! The illustration itself looks really good! I like the colour palette and painting style. However, if you're adding cast shadows in areas such as the neck and a bit of the nose, I think it would be ideal to add shadows to other areas as well, such as the side of the head, the hair, the eye area, etc. This will help give the portrait more volume. Keep the direction of the light in mind. I think the light is coming from the right side based on the shadow on the neck, although the shadow on the nose suggests the opposite... Choose a light direction and make sure the cast shadows follow it. Hope this helps!