In Moodle, it is usually preferable to grade activities through the built-in interface for the activity as opposed to the gradebook for the entire course.  For example, it is possible to enter grades, feedback, and upload a feedback file for a Moodle assignment in the assignment activity itself.  It is not possible to enter a feedback file if you enter the grade directly in the gradebook.

You can tell if the grade for an activity has been directly entered into the gradebook because it will appear with a yellow shaded background in the gradebook.  Grades entered through the activity will appear without the background color.  

Erasing Grade Overrides from the Gradebook 

If you have entered grades for an assignment or other activity directly into the gradebook, you will find that it is not possible to change the grades from within the activity itself.  To correct this issue, go to the gradebook, turn editing on, and click on the Update icon for the grade entry in question.  Uncheck the Overridden checkbox and then click save.  This will now permit the grade and feedback to be entered from the assignment grade screen.