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Student rosters in Canvas are not updated immediately, but rather through a script that IT runs multiple times throughout the day. If the student registered for the course very recently (i.e., a few hours ago or in the evening), it may not be reflected yet in Canvas.
Additionally, if the student is wait-listed for your course or has not officially registered for it, that student will not be listed for you until you s/he officially enrolls in the course and/or is no longer on the wait-list.
If none of the above applies to you, and a student who is appearing on your myDU course roster is not appearing in Canvas, submit a Support Center case with Information Technology.