Have you checked if the following setting is set to 'Enabled': grade_report_showonlyactiveenrol. This may be the reason why only part of the results are visible, especially if your users have an enrolment of which the end date lies in the past.
You can check this setting in Site administration > Grades > Report settings, (https://url.ofyour.moodle/admin/category.php?category=gradereports)
You can check this setting in Site administration > Grades > Report settings, (https://url.ofyour.moodle/admin/category.php?category=gradereports)