by Emma Richardson.
It would normally be by the save changes or perhaps on the next screen. It is an option to turn on in Edit Settings for the assignment.
Other possibilities could be that those showing up are from people that are not actually students in the course - perhaps they have another role that is not graded? Or a grade has been manually entered and then cleared out for those students but the overridden box not cleared?
Try logging in as one of the students and go to the assignment and see what is shows there...