There are several things that could be the issue. First, ask the user to confirm that they've given your app permission to receive Notifications. Some users have this turned off by default or they don't accept the permission to receive notifications when they first open your app.
Second, push notifications on iPhone require a valid certificate to send the notification to Apple. These notifications expire after a year so if you're near the yearly anniversary of when you signed up for your app, it could be expired. Login to your Apple Developer Account and make sure your Push Certificate has not expired.
Third, check the Blogs Editor to make sure there is a green checkmark next to the Blog Post. This indicates that a push notification has actually been sent.
Lastly, if the user has switched phones recently, the notifications may still be going to the old device. You should suggest that the user delete and download your app again to make sure the notifications are going to the new device.