If a student joins a class late, CommonLit automatically assigns them all assignments created in the last 30 days. However, a new student will be automatically exempted from any assignment created more than 30 days ago.
To re-assign an assignment to a student who has been exempted from it, go to the Assignment Report (My Classes > Assignments > find the assignment > click Dashboard > Assignment Report). Scroll to the Exempted from Assignment list at the bottom and use the checkboxes to reassign the text to the student.
Please note: another issue could also be the assignment's start date, if the start date is in the future. Please see this FAQ for more information: Why can't my students see their assignment?