The form, proportion, edges, and form are great. The only part of the form you missed is the halftone planes. You know, the halftone is the plane which is not directly lit by the light source, but they still reflect some of the leftover light, this is because the halftone plane is moving slightly away from the light, but not completely that they become the shadow. I see and guess from your work that the light source is coming from the front top of the head. This light will create halftone plane by the sides of the nose and also by the side of cheekbones. You see, as of right now, it seems to me that the nose and cheekbones are rather flat because there's no halftones in them. Especially for the cheekbones though (because I see that you've tried to include a bit of shading by the side of the nose, even though it's rather thin), it's the obvious first thing I noticed from your work. The cheekbones have front and side planes, so if the light is coming from the front top of the head, then the side of cheekbones should be in halftone, fading away into shadow as they move towards the back of head.

Hey Pedro, Nice drawing. I think what's happening here is that you are doing more of a finished drawing then a lay-in. A lay-in should be made up of 2D shapes, there shouldn't be core shadows or more than one value. A traditional lay-in is made up of straight lines, including the line that separates the light from the shadow. Once the shadows are all mapped out, you then fill them in with a light, even value. After that, you start to refine your lines and shadow edges and so forth. You still have a nice drawing though, so keep it up :)