1. Check the volume is on and set to a suitable level.
2. Check if the notification settings are enabled for the app and allow notifications in your device settings.
3. Ensure you have an active data connection and the app can connect to the server.
4. Try logging out of the app and logging back in.
5. Force stop and clear cache of the app.
6. Restart your phone.
7. Update the app to the latest version if possible.