If you have inadvertently modified grades directly in the gradebook (rather than though the activity) they override all other grade entries and can no longer be modified through the activity. An overridden grade is highlighted in yellow as per the image below in the Grader report.
Enter the Gradebook by clicking on the Grades link in the Navigation menu
In the View tab, click on the Single view tab
You can either: (1) select an assessment from the Select a grade item… drop-down list to show all the grades for the item, or (2) select a student from the Select a user… drop-down list to show the grades for all items for a single student
Grades that have been overridden will show a tick in the Override column
To remove one override, uncheck the box; to remove several overrides at once, click the None link. Then click the Save button
The override/s will be removed and you can now enter the grade/s
through the activity