My students can't see their quiz grades. What should I do?

Quizzes have some additional visibility controls not found within other modules.  Usually an issue with these settings is what prevents students from seeing their quiz grades. To access the specific visibility settings for a quiz, turn editing on and to the right of the quiz select Edit and then Edit settings from the dropdown menu.  Scroll down to Review Options.  This section allows you to control what students see and when:

Points, refers to students scores.  If you uncheck this option for a particular time period, students will not be able to see their score at that time.  For example, if Points is unchecked in the second column then students will not be able to see their score immediately upon completion of the quiz.  If Points is checked in the third column, students are able to go back to the quiz after they have exited the quiz and view their score as long as the closing time for the quiz has not been reached.  

If you have set a close time for the quiz, the last column will be any time after that time has passed.  If you don't have a close time then this column is disabled.