Ok, I will try to clarify it as much as I can, as I don't understand moodle that well. First of all, thank you for your reply. Yes, it was migrated from one server to another. Some quizzes are missing grades, but only for some students, not all of them. Some assignments' grades are also missing. Again, only from a couple of students. The grades are missing from both places, it's as they never took the quiz (the attempts are full again, 2/2).
I'm not the person who migrated it and I can't contact the person who did it, that's why I'm in this mess. I don't know if there's any way to recover the grades of these students or if, at least, I can see a list of it and integrate it manually.
Thank you very much.