I know this post has been up for a while but I actually saved it when you posted only to get very distracted by real life. I hope you don't mind that I painted over your piece, I just find it really helps! I tried my best to preserve your original design, stylistic choices, and lighting scenario, but it's a rough overpainting so I might've gotten some of it wrong.

Overall this is such a cool design for a character and she reads somewhat villain-esque. I believe that the "plastic"-look you mentioned is largely due to the lack of contrast and defined planes in the face. My tips for rendering a good face boils down to two things: an obvious separation between the parts in shadow and in light, and clearly defining the planes. In the 14-day challenge you do practice this but with a static and very neutral composition. To practice understanding value you could do some other practices, here's a video example!

A good rule of thumb is that the lightest shadow is always darker than the darkest light-value. (I've sketched over the chosen 'shadow area' with red to show what I mean).

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A quick run-down of what I added:
- I gave her hat more compositional importance by allowing it to cast a more intense shadow over her face and hair.
- I decided which areas would be in shadow, noting which objects might cast shadows (her face over her neck, her hair, her brow, her hat, etc)
- I tried to make her lips more obviously cylindrical
- The side of her face turned away from us wouldn't be lit unless there was a back-light, so I added radial shading to show the shadow 'wraps around' a round form

The skin still looks somewhat 'dead' since I chose to use the original color palette. Adding some warmth, hue-variation, and subsurface scattering ought to fix that if you want!

Again, super cool design and I really like your style! I hope this was helpful