useful tips that I’ve found help with my oil paintings. Assuming you’re working from a reference.
Take a photo of your painting and switch it to black and white using a filter on your phone. Now, set your reference to black and white. Do they look similar? often, we get our values wrong which will make a painting look flat and unrealistic. If your reference and painting dont look extremely similar (lights and darks are roughly the same), then your values are wrong.
using your phone, flip the reference photo and the painting of your photo horizontally (on iphone, click the crop button and then look at the top left of the screen for the mirror button). We usually get used to the way a painting is supposed to look which can cause us to overlook errors because we are ‘used to’ them, so when you mirror the painting it will be like you are looking at a brand new painting. you might have an easier time spotting errors.
Personally, I think you are lacking a range of values which is making the painting look flat. Also, the neck is too smooth and dark, which throws off the painting. it looks like the light in this image would be coming from roughly the direction that her nose is pointing in, which means the neck should be a similar color to her face with a shadow cast from her hair.
edit: by lacking a range of values, I mean that your values all seem to be roughly in the center (no dark darks, and no light lights). this makes the picture look very flat and unrealistic, which is hurting you because I think youre going for realism here.