@Shay , Thanks for the question!
Notification delivery can fail at any of the four stages in the push notification process (client, application back end, Notification Hubs, and the platform's push notification service).
This article identifies why notifications might get dropped or not be received by devices. It also explains how to determine the root cause.
Diagnose dropped notifications in Azure Notification Hubs
Push notifications depend on third-party Platform Notification Systems such as Apple's Push Notification Service (APNs - as in your case) and Google's Firebase Cloud Messaging (FCM), there is no SLA guarantee for the delivery of these messages. After Notification Hubs sends the batches to Platform Notification Systems (SLA guaranteed), it is the responsibility of the Platform Notification Systems to deliver the pushes (no SLA guaranteed).
Hope this helps!