Also when an alert has been viewed in pop-up area it will only be retained in the new alerts pop-up area for 3 hrs before expiring. It will still be visible in the "show all alerts" but just not with the new 'red' notification. Viewed alerts in the "show all" area are retained for 4 days.
Users can opt out of alerts but hovering over the Alerts button and selecting "Alert Preferences". So check here and make sure everything you want to be alerted for is ticked.
Last edited: Dec 21, 2017