While by no means a Moodle expert, this one drove me nuts as I believe it was a change from 1.8.
What happened to me was that I used the Grader Report, not Quick Grading on the Assignment page, to enter a bunch of scores. Moodle treated these as an Override and now I could no longer change them on an assignment page. I could spot them easily later on in the grader report because they were shaded (#1). The only way I found out to turn off the override was to click on the edit icon (#2) and then uncheck override. I also found for some reason that I had to do this TWICE as it never saved the change the first time. I don't know why that happened. Doing this was very tedious and time consuming as I found no way to clear these overrides in a wholesale manner.
The moral I learned was never enter grades via the grader report.
