If you added a new student to a class with preexisting assignments, they will not be able to see those assignments on their Timeline. This happens so that students who may enter a class midway through the year are not overwhelmed with late assignments.
To easily send assignments to an individual student, simply go to your Timeline and click "Everyone." A dropdown menu will appear. Click the desired student.
You are now looking at their Timeline. Simply drag and drop your already existing assignments in Planner onto their Timeline. Happy teaching & learning!