It is possible that a quiz submitted by a student was scored, as can be seen by viewing the attempt in the quizzing tool, but the grade was not exported to the gradebook.
Either the quiz has not been properly set up for auto-exporting to the gradebook, or a database or other internal error occurred that prevented the grade from being exported to the gradebook.
To resolve this issue:
Go to the Assessment tab in the quiz and see if the quiz is associated with a Grade Item and the boxes allow automatic export to grades and allow attempt score to be seen immediately upon completion are checked.
If the above boxes are checked for auto-exporting, report the problem using the Problem Reporting Form. Be sure to specify the user names of the students whose grades didn't auto-export as well as the quiz and complete course name.
Export the quiz scores into the gradebook manually for those students whose grades didn't auto-export:
Select the quiz from the quizzing tool.
Disable auto-exporting temporarily to allow manual exporting. Uncheck allow automatic export to grades from the Assessment tab and click Save.
Select Grade from the drop-down near the top.
Check the boxes by the attempts for students whose scores didn't auto-export.
Click the Export to Grades button near the top.
Note: If you don't see this button, you forgot step 3b above.
Restore auto-exporting by checking allow automatic export to grades from the Assessment tab and click Save.