When you first send an assignment students may need to refresh their browser.
If a student requests to join a class after an assignment has already been sent, they will be unable to access the assignment.
To remedy this, go to your Timeline and find the assignment. Click on the assignment, the grading page should open. On the top right there are three little dots, click that then "Edit." The create assignment window should appear. Click where it says "Assign To" and add the appropriate students.