Hm - I'd maybe play around with the palette? The skin tones (especially the shadows) look kind of washed out and grey.

Semi-realistic portraits can be tricky in PA for precisely this reason, because you don't have access to the subtle color gradations that make faces look alive (hints of blush in the cheeks, yellow in the forehead, etc) - there's an attempt to do that here with the slightly reddened shade of the base tone you're using for the ears, I suppose!

Oh, and I don't know what's going on with the bit of shading above the lip, but that looks kind of awkward - I think it's trying to approximate the little dent beneath the nose, but it just looks a little awkward instead.