Following on from our IRC convo, I went on and recreated the nose )along with the chin, lips and up around the cheek that you'd already seen) and a bit more of her left cheek for you. What I've changed from your own latest version is:
* I've pretty much cut out the dithering in the area I've done.
* Simplified the shapes, reducing all of the unnecessary crammed details - for example it's impossible to pixel the texture of the lips onto such a small canvas!
* Given the skin more colour, it now looks more vibrant
* Given the shades more contrast, especially around the darker areas.
* Given her more natural cheek shapes. She looked more gaunt than Peter Cushing on your version.
* Smoothed the lines and anti-aliasing.
These were the areas where you were going wrong. Some of your linework was a bit messy. Choosing a good palette and having good linework are the two most important aspects of pixel art (and any art for that matter), they are the basic essentials. You must, above everything else, make sure that your curves are smooth and the anti-aliasing has a smooth blend between shades. There's one point, which I pointed out to you in the IRC, near the hair where it is criminally pillow shaded. You must avoid this at all costs!
The structure of the face around the cheeks and side of the lips was very inaccurate. There was far too much immediate, very dark shade there. The only way to really understand this is to study people's faces and use reference pictures.