To ensure the prompt delivery of WhatsApp message notifications, your phone must be configured properly. Here are some tips on how to configure your notifications:

• If battery-saver is enabled, notifications don't get delivered.  Please turn battery-saver off if you expect to get notifications. 
• Notifications on Windows Phone require a strong carrier data connection.  
  • If your phone has a bad internet connection, or you spend part of the day without strong cell coverage, it may take some time before WhatsApp is able to reconnect to your phone and send you notifications.
• Ensure WhatsApp is pinned to start. That way, if you miss a notification, you can still see an unread message count on the live tile. 
• Ensure your ringer is on and the All other notifications box is checked under Settings > ringtones + sounds.
  

Cheers,
WhatsApp Support Team