WhatsApp: Frequently sleeping I have three Android devices.

Lenovo A7000 with Stock ROM Lollipop 5.0
HTC Desire 620G with Stock ROM KitKat
Samsung galaxy Tab 2 with custom ROM (Resurrection ROM) Lollipop 5.0.2.

In all these devices the WhatsApp sleeps frequently, when the screen is Off. I mean when I on the device screen and use WhatsApp I am getting messages normally. But when the screen is off after a short span of time, it stops receiving messages. I need to turn on the screen and open WhatsApp to start getting the messages. Then bunch of messages will be received.

I am facing this problem in all the three devices in both Wi-Fi and Mobile networks. My Wi-Fi network is fast enough (40 Mbps) to receive the messages at instant and mobile networks are decently fast (>3 Mbps).

In WhatsApp Web I am getting Phone Not Connected yellow messages when I turn the screen off. After researching in the Internet I learnt this may happen when the device enters power saving mode or when Background data is restricted. But in my devices I never turn on power saving mode and never restrict background data too.

Any other reasons? For this reason I have changed screen timeout to 30 minutes and left the WhatsApp open. Is this an app bug or some other problem?
It's common. I've tried all sorts and still don't receive messages until I manually turn the screen on. Only thing I can think of is using wake lock power manager https://play.google.com/store/apps/details?id=eu.thedarken.wl
May use a little battery but will fix your problem