There are many reasons that you could not receive notifications.
1. Notification is turned off. In this case, please check if you turn on notifications on the device settings and the app settings.
2. Apps are sometimes killed every X minutes by Android system when in the background for battery optimization
3. Unstable network connection. In this case, please check your network first. Also, we send the notification based on Google service. If you could not access Google, the notification will fail.