Why can't I change a particular grade in some activity?
If a student's grade in an activity , has been changed in the Gradebook, it can no longer be changed in the activity itself.
For example, if a grade was entered in the Gradebook for a particular student in a particular assignment, the Edit page for that student in that assignment (reached by clicking on the Edit link for that student in the View/grade all submissions page for that assignment) shows This grade is overridden or locked in the gradebook, instead of the usual window for entering a grade.
In the Gradebook itself, in the Grader report page, the gradebox for that assignment/student is highlighted.
In the Edit page for the grade (reached by clicking on 
in the gradebox when editing is turned on), either Overriden or Locked is checked.
They can be unchecked to be allow the grade to be changed in the activity itself.