Sometimes students are allowed to have a certain amount of submissions per assignment. If the button to add another submission is not showing up for the student we can go in and manually allow for another submission by doing the following:

1) Logging into the specific course as admin.

2) Viewing all of the submissions for the specific assignment. 

3) Clicking on the specific submission that the student has already submitted.

4) Scrolling down to the bottom and there is a button to allow for another submission.