In some instances a quiz results won't display in the gradebook and it's difficult to determine why. This issue is really a request for an improvement in the gradebook UI.

Go into a quiz from the Grader report. Hit the "update this quiz" button. If an instructor has deselected the "Scores" checkbox (in the Review Options section), the grade will not be visible to the student. The maintainer of the gradebook may try to make the grade visible from the report itself, but it will always go back to ticked because the quiz module is preventing the score from being displayed.

Seems that best remedy would be to turn off the option in the report if it's already off in the quiz.