Grades do not save
There is a known bug when entering grades into a Moodle course gradebook that revolves around the fact of having more than 15 people on one page. If your course has more than 15-20 students, the grades of the people after this will not be saved when you update them.
The current workaround is this:
Settings > Grades, then pick Settings > My report preferences > Grader report, set Students per page to fifteen or less and Save.
Having pages of 15 or less will solve this issue.