Currently, submitted assignments are only able to be reassigned before they are graded. This is also the case when Grade this Assignment Automatically is selected when creating an assignment.
To provide a student with another chance at performing an assignment which has already been graded, you will need to remove the student from the assignment and then assign again. To edit an existing assignment, open your gradebook and click the edit button for the specific assignment.
In the resulting window, choose Edit Timeline and Students.
From the edit page, choose Assign to Some Students to bring up the student roster.
From the roster page, uncheck the student(s) you whose grade you would like to remove and re-assign to. Click Save.
Click the Next arrow to complete the changes.
Once finished, return to the edit screen for the assignment and follow the same steps to re-select the student(s).