Turn on WhatsApp notifications
This could be an easy and obvious fix to the problem. When you first set-up WhatsApp, you may have not allowed the app the required permissions for notifications, either on iOS or on your Android smartphone. Solve the problem with these steps:
Turn on notifications for iOS devices
For your Apple iPhone or iPad. Go to Settings. Go to Notifications and then select WhatsApp. Then toggle Allow Notifications. Done.
Turn on notifications for Android devices
On your Android device, just do a long tap on the WhatsApp icon and open the app info. Next, pop to Manage Notifications and toggle to Allow Notifications.