There are multiple reasons why your students may not be able to view their grades: They include:

1) The quiz column may not be released to the students.
2) The quiz settings may not be set to release the students' scores.
3) There may be paragraph questions in the quiz that need to be marked by an instructor or grader before the quiz score can be released.

1) When a quiz is created a column for that quiz is automatically created in your Student Management area. As a default the column is not released for the students. You must release the column for your students to see their marks.

While logged in as designer click:
Homepage > Control Panel > Manage Course > Manage Students > Manage Columns

You will see the status for each column in the Released row of the Column Editor. "Y" means that the column is released and the students can view their marks. "N" means the column is not released and students will not be able to view their marks. The Hidden toggle works in a similar fashion, except that if a column is Hidden it means that it does not display on your (designer's) Student Management page.

To change the status for any column select the box above the appropriate column, or columns, under Options - Release columns,click the dropfown box, select 'Yes', click 'Go'

2) To view the settings for your quiz go to the Quiz/Survey Management area of your course site, click on the quiz hyperlink will bring you to the Quiz Editor.

Click [Settings] on your designer toolbar.
Beneath the Results section you can see the settings for Student Score release. If it is set such that the students score is not released you will need to change it to "release the score once the quiz has been submitted" and click [update] to have this change saved.

Return to the Quiz/Survey Management screen.
Click on the Submissions link for the appropriate quiz.
Click [Mark All], and then [Regrade] on your designer toolbar. This will regrade each students quiz submission so that their marks will now be viewable. Please note that if you have manually overridden any students' grades, the "Regrade" function will reset the grades to their original values.

3) Paragraph questions must be marked before the quiz results can be made available for students. To grade paragraph questions go to Quiz/Survey Management area for your course.
Click on the Submissions link for the appropriate quiz.
To view a student's quiz click the number link in the submissions column.
Grade the paragraph question, or questions, by entering a grade into the appropriate mark box and clicking Update to save these changes.

Now that these questions have been marked your students will be able to view their grades provided that the previous two conditions have been met.

You can have the grades show by enabling them to be visible in Course
Management -> Student Management -> Column: Column Editor :

The table indicates a column's current status. Toggle Released to "Y" to show this column to your students.

There is a small design flaw I recently discovered in the quiz grading process. According to the design of the quiz module, students need to click on "View Results" to trigger the grading process. If they do not click on that, the quiz is not graded. (Also, as one would expect, if a quiz has paragraph questions, WebCT cannot automatically grade it.)

I asked Dr. C. (WebCT gurus) about this problem and here is a summary of a workaround they suggested.

Some students are not curious enough to actually click the "View Results" button after completing a quiz, so you can make it automatic by pasting the following Java Script into the submission box in the designer's Quiz Settings display. The code has only two lines within the <script> tags. The first line begins with "doGrade," and the second line begins with "alert." The alert is included to avoid the problem of students quitting WebCT (i.e.. closing the browser window) before the quiz results fully display, thereby aborting the grading. All of the quotation marks in the doGrade line are 'singles,' not "doubles."

<script language="JavaScript">
doGrade('','SELECT'); alert("To ensure that your quiz grade is recorded, be sure to allow the results window to display completely.");
< /script>