When you first send an assignment students may need to refresh their browser.
If a student joins a class, they won't be able to see assignments that are already due.
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.