I have recently bought a Samsung Galaxy S4, Android version 4.4.2, and for some reason I only receive my WhatsApp messages when I open the app. This only happens when I am on 3G. When connected to a WiFi network, there is no delay at all.
I have uninstalled and re-installed, I have checked the restricted data setting (it is not ticked).
What did I miss?
Samsung phones have an option to sync data in the background, disabling it, saves data, as updates are only checked for when an app is open (email/im/whatsapp etc). You need to enable that (see green icon in image below) to let your apps periodically check for data.
Screenshot (click for larger variant)
Could this also be in your "Smart Manager". When you click on "Battery", it says "Optimising". When your apps are selected, it will only updates when you actually opens the app. And it can also be your "power saving mode" problem in smart manager. Once you click on power saving it shows you restrict all background data, this should be disabled.