If you added a new student to a class with preexisting assignments dating back over a week, 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 Class Content onto their Timeline. Happy teaching & learning!