I'm not receiving Whatsapp notifications in proper time, I only find out new messages when I open the app, but in the meanwhile nothing notifies me about anything. I am using Lumia 520. All settings checked and enabled. This problem started 3 days ago. Before that all is fine
Did you take any new update of your application? Or it could be a reason of slow internet connection, 2G connection.
There's some problem with Whatsapp itself. I am also facing the same issue, when I am on 3G. The notifications are not delivered properly or on time. While on wifi, they come instantly.
So probably you need to email whatsapp and notify them of this bug/issue. They might help you on this.