If a student joins a class, we automatically assign them all assignments created in the last 30 days. However, a new student will be exempted from any assignment created more than 30 days ago.
To re-assign an assignment to a student who is exempted from it, go to the Assignment Report (My Classes > Assignments > Assignment Dashboard > Assignment Report). Scroll to the Exempted 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: Why can't my students see their assignment?