Let's say I have 20 separate quizzes of four different types. I go to the grade page, I click the "Set Preferences" tab, then click on the "Show Advanced Preferences" button. Then I click on the "Set Categories" tab. Then I add the four different category types and categorize each quiz according to one of these, then I save the changes.
Then I click the "View Grades" tab, and am shown the quizzes in four columns. I no longer see the individual grades. I should be able to see them when I click on the category headings at the top, and most of the time I do. But sometimes I will see grades when I click on one category heading, but if I click on the one next to it, it will not show the grades, as if there were no quizzes under the category.