I have Microsoft Lumia 535. I can't receive anything on WhatsApp when it is not open. When I open the app I instantly receive messages but not when it is minimized or not open.
Is there a shield symbol on the battery icon? If so, the phone is running in battery saving mode. This will stop almost all background tasks and you will not get any notifications expect calls and text messages.
You can toggle this under Settings → Battery saver.
This is probably because of either
- slow internet connection
- battery saver is enabled
- background service is not working