Firstly, some of the values have too much contrast(very dark next to very white) Add more subtle values to create a better allusion of space.Secondly,when blending take as much time as possible using a flat kolinsky brush(espesially small sizes). Be careufull that darks aren't destroyed by the white paint. Then, use the camel brush to smooth the texture of the paint. The correcting brush is responsible for the subtle transitions in tone.
Finally, your darks are applied too thickly. Allow some of the umber underlayer and imprimatura to show throught. This produces a very good effect.
