This situation is produced when you select "Attempts must be submitted before the time expires, or they are not counted" from the options under When time expires in the Timing settings in Quiz administration >> Edit settings.
In that case, if any student fails to submit the quiz before the time expires, their attempts will appear as "Never submitted" and they will not be automatically marked.
The solution to this problem is complicated and involves changing settings in a live activity, we recommend logging in your query in Service Now.
The solution will require your permission to:
Export all quiz scores and the unsubmitted attempts, just as a back-up.
Create a group of those students who have not submitted, restrict Quiz access to the group.
Re-open the quiz in the Timing Section and add a 15-minute grace period.
Set attempts allowed to 'Unlimited'.
Set question behaviour to 'Each attempt builds on the last - Yes'.
Login as the relevant student, start an attempt, submit attempt.
Repeat step 6 for each student in the group.
Change back all settings, including the original date.