I got you some fixes. Hope it works for you !!
Follow the below steps:
WhatsApp > Settings > Notifications
Under Notifications you will find many options but you have to scroll down and select “Pop Up Notifications”.
A Small window will appear with the following options:
1. No popup
2. Only when screen On
3. Only when Screen Off
4. Always show Pop up.
If you want to see a pop up on your lock screen even when the display is off click on Only when Screen Off this will help you to get messages from contacts as notifications on your lock screen.
You can try this too
Go to Application Manager > WhatsApp
You will see Notification and it may be disabled.
Click Notifications > Allow Notifications
Hope any of the ways helps you out buddy!!
— modified on Jun 28, 2018, 9:33:07 AM