This can have two causes. Either no subscription is assigned to the students' class, or the students are not in a class.
Please make sure you assign a subscription to each class. To do this, follow the steps listed below.
Via Manage > Students > Students not assigned to a class, you can find all students who are not in a class. You can add a student to a class by looking them up in the list of unassigned students. Click 'Open' next to their name in the list, choose 'Edit' and click 'Add student to another class' under 'Assign classes'.