by Kojo Nyame.
I had this issue recently with my Moodle 2.5 but it was because the I had not somehow selected or deselected the "file submission" check box in the "submission type" option when you edit the assignment.
When I selected that check box, the students could see the submission button. The help pop up by the check box actually states "If enabled, students are able to upload one or more files as their submission. "
I post this in case anyone may have somehow missed this option being deselected, whether by duplicating an assignment or as I did, by resetting the course after backing up the same.
Kojo