You can't see some students that you know are in the class on the list of submissions within an assignment activity.
A) Student is not enrolled in the class
If you manually enrol students you might need to make sure you have enrolled this student in your class.
From the administration block, click Class administration>Users>Enrolled users
Search for the student in the Search box and click 'Filter'
If you can't find them you may want to enrol them manually, but please note that if you enrol them manually you will have to manage that enrolment locally.
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B) Student is enrolled but their enrolment is suspended
If you can see the student in the enrolled users but the 'Enrolment method' is greyed out, this means that their enrolment has been suspended
The student data (such as assignment submissions, quiz results) still exists in the class but is 'hidden' until they are manually enrolled or their suspension status from the pegasus enrolment code changes and the are automatically re-enrolled.
If you need to retrieve their submission from an assignment or you need to give them access to this class you will need to give them a manual enrolment as a student on the class:
C) The assignment is restricted to a group that the student does not belong to.
If the assignment or section is restricted to a particular group or set of students you will need to ensure the student you are looking for is part of the group
See Creating a Group to find out how to manually add the student to the right group.