Your use of pencil looks excellent on the portraits but I think you're using too many total white areas for the skin to look natural, the drawings look like bronze reliefs. If you want to leave 100% white spots at all (though there's rarely any on skin), using just one for the brightest highlight, often at the tip of the nose, and making the rest of the highlights just almost white adds loads of depth. I don't know how you shade but doing it in layers moving from light to dark values would be the way to go for maximal impression of depth. I sure hope it's not just a style choice I'm bashing. The reference studies look spot on otherwise and the robot on the last post and angry face two posts before are great designs. Keep these coming!