Many people experience issues with WhatsApp notifications since the iOS 11 update.
IPhone users don’t get notifications for new posts or receive the notifications only after opening WhatsApp.
Although the problem is happening with WhatsApp, it’s likely an issue with iOS or Apple’s Push Notification Service (APNS). Notifications are sent to your phone from Apple’s Push Notification Service (APNS) and the delivery is entirely in control of that service.
Despite we think that the problem is not caused by WhatsApp, we have informed WhatsApp about the many complaints. They will investigate and as soon we learn more, you can read it here or on our Twitter (and Dutch Twitter)