Here are some possible reasons and fixes for when users report they are not receiving notifications. Once troubleshooting has been completed, you may test the notifications by opening the app, going to Settings then tap Generate Test Notifications.
An additional thing to check would be the phone data and time. Make sure they are set correctly since Firebase depends on this to both send analytics data and notifications.
Network Issues
There may be issues connecting with Google's servers. Try turning off and turning on your phone's internet connection.
...