I did receive notifications but they are delayed. What is the reason for delay?
Normally push notifications do arrive quite quickly. But there are some rare cases where we've seen them get delayed:
Delays for notifications on Android Devices:
Some home and commercial wifi routers will cause the device's connection to Google's FCM/GCM servers to be closed. The device later re-opens the connection and receives the delayed notifications. (Discussed here: https://groups.google.com/forum/#!topic/android-gcm/Y33c9ib54jY)
Some Android devices can enter a low power mode when their battery is low that may cause slight delays in receiving notifications.
Delays for notifications on iOS devices:
As with Android, some wifi routers will cause iOS notification delivery delays.
Notifications may be delayed if "Restrict background data" is checked for the app, if Wifi-Optimization is turned on in the advanced settings, or if priority mode is enabled.