To resolve the issue,
Tip 1. Check whether your mute button switches to Silence. If it does, there would be no Sound when the notification is created on your iPhone or iPad.
Tip 2. Check whether "Do Not Disturb" is activated. If this feature is enabled on Settings app, calls and alerts will be silenced while your iPhone locked.
Tip 3. Check whether you turn on the notifications for the apps by going to Settings > Notifications. If you turn off notifications for some apps, it won't notice you the apps' news.
Tip 4. Check whether the network is working in a good condition. If it isn't, apps won't update the news and send notifications. Just re-connect your Wi-Fi or Cellular network. Or reset network settings.
Tip 5. You can restart your device, which is one of basic tips to fix any iOS issues.
If you still face the issue, You need get the latest version of app from Appvn download for iOS.