I totally agree with what @Patrick Bosworth said about the lighting, and separating the planes of the head. I wanted to talk about your drawing. Because you didn't post the reference, I am going off your drawing. The biggest thing I see is the lack of structure, and because of that it lacks a solid feel. For example: the torso and the neck don't feel connected. the ear is way too high on the head. the eyes are flat and don't feel like they are set into the head. the hand is drawn pretty well, but is too small. By working out all the elements and structure before doing your finished drawing and painting, will create a solid base. I did a quick sketch to show you what I mean, I hope it helps :)

Nice work! You could start to take this to the next step by adjusting your values, and working in some lighting and to help separate the planes of the head. You're setting up some lighting in the top hat by showing the wrapping of light to dark around the side of the head, but you didn't connect the shadow down on the side of the head and neck which flattens the face a bit. I roughly added some shadows to exaggerate some planes so you can see what I mean. Check out this video from Marco Bucci talking about portrait painting, and lighting the head. Keep it up! https://www.proko.com/lesson/painting-the-head-from-imagination-lighting-without-reference-with-marco-bucci/discussions