If the mobile device you were using at the time had a weak or intermittent internet connection (low cellular or Wi-Fi signal), it can affect the alert notification being received. Another possibility is if your mobile device was in deep sleep or hibernation mode, which it can do if it has been sitting idle for a while and/or if the battery is low. This too can affect an alert notification being received. With both of these possibilities, however, you will still be able to see the alert when you manually open the app.