by ben reynolds.
Notifications only go out once. (An exception may be the whole submit draft, receive feedback, revert to draft, submit draft, receive feedback, revert to draft type of Assignment, but since we don't use it, I don't know for sure.)
The workaround is to select all students in messaging and send them a message that the feedback has been updated.
Too, I would inquire to be certain that *all* students didn't receive the first notification.
And, I note this about notifications in 2.6+ about Assignment notifications: "If enabled, graders (usually teachers) receive a message whenever a student submits an assignment, early, on time and late. Message methods are configurable."