My students can't see my comments on their quiz question answers ...
To allow students to view feedback comments on Essay style quiz questions:
Click "Review Options" in the editsettings area for the quiz
Tick all of the boxes under the "Later, while the quiz is still open" option. You can also tick the options under the "After the quiz is closed" option if you want the students to still be able to view the feedback comments once the quiz has closed.