I am using the latest Moodle 1.9, mostly with Hot Potatoes quizzes delivered through Quizport.
I can edit grades manually on smaller courses, and in the first rows of the gradebook for large courses.
But in a course with around 50 students, I cannot enter grades for students below about the 25th row of the gradebook.
When I try, everything looks normal, but the changed (or new) grade does not record. After "Update," the grade I entered disappears from the gradebook.This affects only students in the bottom half of the gradebook.
I speculate that the large number of students, combined with a large number of exercises, overrides some limit.