Good job.
I think you need to establish a light source and add highlights/lighter values on the side facing that and shadow/darker values on the side turning away from it. Here's a really old, lo-contrast and not very well-rendered shading study I drew once that still might help you with placement of values.
A good practice routine and warm-up I use is to draw (pencil on paper) the three basic shapes - sphere, cube, cylinder - establishing a main light source and shading them in tone. Hope that helps