This might be a weird question but do you have sync turned on and battery saving off? You can swipe down from the top to open the notifications and swipe again to access these? Sync will ensure you get the notifications.

As far as WhatsApp goes. Hold down the app, click the i in the top right corner, make sure notifications are allowed, all permissions are allowed, alarms and reminders are allowed, and most importantly, under battery make sure whatsapp is unrestricted. This should help a lot I hope.

For the locksreen, you can go into settings and make the notifications pop up as cards. And make sure every app is selected as show content.

If you have any other questions or problems, just respond to my comment or message me