Hmmm. I can give few advices. Of course I think that you got these ideas yourself. First , make fingernails brighter than skin, second, if you have some kind of large mirror or even photo of random human , check it out when drawing new scenes to help yourself with proportions , because it's easy to mess it up when you draw a lot of scenes. Of course it only my unprofessional opinion and I don't want to insult you in any way. But if you have an opportunity to try these advices , then maybe they'll help you. Wish you luck at improving your skills.