Just in case this helps - could it be that a 'resubmission' has been allowed for those assignments? If so, the previous submission is relegated to a collapsed 'history' section at the bottom of each page (the main page for the student, the individual grading page for the teacher).
( http://www.synergy-learning.com/moodle-2-5-improvements-assignment-resubmissions/ has some screenshots of this in Moodle 2.5)