You shoud is more of a suggestion, like "you should do your homework" or saying "you should be doing your homework" means that you want them to do their homework, and that they should be doing it in that moment, but it's not a very strong way of saying it. "you're supposed to be" is a stronger expression, I find it's more accusation. My mother could say both to me, but she would use "you're supposed to be" if she was mad at me, not just annoyed. You're supposed to be could also be an accusation after you've been lied to. If you're having a party at your house because your mom is out of town, and she comes back, you would say "you're not supposed to be back until tomorrow!" You're supposed to be is an assumption you had about what someone was doing, while you should be is a direct suggestion.
Good luck, I hope that made sense!