1) Check whether you granted push alert to your app. In iOS go to Device Settings > Notifications > App in the Air > "Allow Notifications" should be turned on. If you don't see App in the Air in the list of apps, go to App in the Air app > Trips screen > Select any upcoming flight > Open Alerts widget > View > Turn on alerts and allow alerts.
2) Check if you have alerts turned on for this flight: Open Alerts Widget on the Flight Feed > View -> Turn on alerts and allow alerts.
3) Check alerts settings. More on alerts settings can be found in this article.