Hello
I have the same problem: no notifications from whatsapp. This morning I did a hard reset. It didn't work. I've restored my iphone (4) and I have restored only the contacts and calendar data (I cant lost it). Then I have reinstalled the app. It still doesn't work.
It only works if I have the screen active. Passing two minutes from the phone blocking, the connection in whatsapp get lost and I can't get any notification. When I unblock my phone and open the application once again, it gets connected and then all the messages are retrieved.