When a Teacher creates a Turnitin assignment for the first time within an onCampus course, a Turnitin class is also created for that course on the Turnitin database. This class has a default end date of six months from the creation date. It doesn't automatically pick up the class length from the setup in onCampus. Once the end date is reached, the class becomes “expired,” which means any submissions in the class are read-only. When opening a student's submission, you will see this message:
The way to "push out" the expiration date of the class is to create a "fake" Turnitin assignment in onCampus with a due date in the future. I would suggest creating an assignment with the last day of the grading period as the due date. Once you save the assignment for the class, you can then immediately delete the fake assignment. Existing assignments should then revert to one you can review and grade.