First, make sure the learner has met the mastery score requirement. They could have simply failed the course.
It's possible the learner did not close the lesson properly. Learners must use the "Exit Course" button within the lesson for the quiz results to save properly.
The learner could have clicked the "Complete Survey" button (at the end of the lesson) and was redirected to a separate page and then closed the browser from there without going back to the lesson and using the "Exit Course" button, which must be done for the system to save results.