In order to receive lead notifications, you must have notifications enable for the Connect Now App. During the setup process for the app, you will be prompted to make sure that "notifications" and "locations" are turned on for the app.
If at any point notification are turned off, you will be notified the next time that you log into the app.
With notifications turned on, you will receive lead notifications even the app is closed.
There can also be delays in receiving notifications caused by a variety of issues:
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 custom versions of the Android OS have a power-saving setting that can cause notifications to be delayed or ignored. For instance, Sony Xperia devices have a "Stamina Mode". (Discussed here: https://talk.sonymobile.com/t5/Xperia-Z3-Compact/Notifications-not-Working-When-Phone-in-Sleep-Mode/td-p/879641)
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.
"Low Power Mode" is enabled. If in your Settings > Battery > Low Power Mode is enabled, this will reduce the rate at which notifications can be received or turn them off completely.
WhatsApp has a good list of instructions on troubleshooting delayed Android and iOS notification at the bottom of their FAQ page here: https://www.whatsapp.com/faq/en/android/20887936