Push Notification is not 100% reliable and it will not guaranteed that you will receive it 100% of the time.
Quality of Service
Apple Push Notification service includes a default Quality of Service (QoS) component that performs a store-and-forward function.
If APNs attempts to deliver a notification but the device is offline, the notification is stored for a limited period of time, and delivered to the device when it becomes available.
Only one recent notification for a particular application is stored. If multiple notifications are sent while the device is offline, each new notification causes the prior notification to be discarded. This behavior of keeping only the newest notification is referred to as coalescing notifications.
If the device remains offline for a long time, any notifications that were being stored for it are discarded.
If you have any important message that you want the user to receive, I would like to suggest you to try Local Notification.