Reason 1: Force Stopped
When an app is in a "Force Stopped" state most events including FCM/GCM messages for push notifications will not be received.
An app can be placed in this state in the following ways.
- From Settings > Apps, "Force Stop" is pressed.
- Long pressing the back button on some devices.
- Using a 3rd party task killer like Greenify.
- App is closed on a Huawei or Xiaomi device due their custom Android tweaks.