For Push Notifications to be received the user has to give your app permission to send Push Alerts.
Permission is automatically granted on Android devices. Apple will prompt the user when they first load your app for the permission to be granted.
From time to time there are issues with the registration of Push Certificates (the technical back-end requirements), which will require us to look into it further. This is usually a quick fix - please contact Support if permissions have been granted on the mobile device but the problem persists.