Some problems I already see is how heavily pillowshaded the hair is. I advise you to take reference from various other mugs and see how the hair is shaded in comparison. There's also some heavy banding with some hair strands, which is another huge problem.
Another confusing inconsistency is her shoulder and the proportion of the hair. It seems to lack any depth into showing that the hair is over her shoulder, and instead looks like it's eating into her neck and not the shoulder, which looks really bizarre. You'll either have to fix the shading there, or add more hair to cover her shoulder a bit more.
I could go into a myriad of different reasons, such as the clothes and other shading, but I believe this should be enough to get the point in. I pray this helps in fixing your portrait.