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 left there is an Edit button, click that to "Edit." The create assignment window should appear. Click where it says "Assign To" and add the appropriate students.