When you click the notifications-icon, there could be a botton in the bottom saying "See all" or something. Then when you click, you'll see both old and new notifications - maybe just 1-2 days back? Sometimes I think mine don't "pop" up, like when one of my papers are graded, I don't get a notification, or it disappears. Maybe I'd be able to find it, if I had the "see all" button.
Customer support service by UserEcho