Whatsapp messages not delivering instantly

When my phone is idle for sometime (five minutes approximately) and in screen switched off mode, the whatsapp messages not delivering. when I turn on the screen and open the Whatsapp application I an see the refresh button activates and delivering all messages. What can I do for getting message notifications even when the screen is off or in screen lock mode. I have set my screen lock to "never" still same situation. No of notifications in screen lock to set t maximum (10) still same situation.