This issue is usually caused by the student forgetting to click Submit All & Finish. Open the assignment with the missing grade. If it says Continue, have the student complete the assignment and click Submit All & Finish. The grade will now be recorded.
If it does not say Continue, the student may have forgotten to select Save without Submitting, or the internet connection may have been lost. Since the student should be solving every problem on paper before entering their answer, they can easily fill in the answers then click Submit All & Finish and the grade will be recorded in the online grade book.
Watch the How To Monitor Student Progress video. It provides detailed information on this issue.