Yes, I have seen the first two quite often.
If the cells are locked (overridden) by someone using the manual grading in the grader report. When the editing is done in the grade book instead of the quiz it will turn the cell color. We had a theme that wouldn't show the color change in every other cell, so we had to check by hand. We had to turn editing on, click on the settings button and look at the overridden check box on the description. Unchecking the overridden check box and saving the changes usually took care of things. If it is just a few students in the course, this may be the problem.
In the category, go to the settings section of the grade book to categories and items and check category setup for the category the quiz is in and look for the locked check box and make sure it is not checked.