If a coordinator denies a student's exam request that has been waitlisted and later decides the student should be registered for that exam, the coordinator will need to reverse the denial. You can do so by going to Report Center > Students > Manage Student Requests > Manage Denied Waitlisted Students.
To add a student/exam back to the wait list approval page, check the box and click Place Back on Waitlist. The student/exam will then show under "Approve/Deny Waitlist" in the require action section of your Dashboard.