Assignment module FAQ
From MoodleDocs
Why can't I edit grades for assignment submissions?
If you edit assignment grades directly in the gradebook in Moodle 1.9, an "overridden" flag is set, meaning that the grade can no longer be edited from the assignment submissions page.
If required, the flag can be removed by turning editing on in the grader report, then clicking the edit grade icon, unchecking the overridden box and saving the changes.
See also
- Using Moodle Assignment module forum
