Gradebook

I seem to be facing the same problem in other subjects, these are subjects where Am the only one enrolled in as a teacher. It's the first time we are first with such a problem and the only thing we have done differently is to allow self enrolment via an enrollment key. Could this be the cause or is it some bug in moodle. And to avoid to avoid disadvantaging the students I had to advise teachers from using the gradebook to export marks just in case there are more students with missing marks.