Not receiving push notifications from Whatsapp?

I have an Iphone 4 and a have a problem with the notifications. I stop receiving notifications from Whatsapp if I exit the application, and i have to always go back into the application and check it, to receive them. I don't understand why, because it's not a problem with the settings, the notifications for this application are turned on. When the phone is charging, it appears that the application remains in the background and i keep receiving all the notifications. What is the problem then? What can i do to receive the notifications all the time and not only when the phone is charging? I thought maybe the application exists to save the battery and that's why it doesn't when i charge my phone. I tried the solution that i read in a discussion here, that supposed to change the DNS field for the wi-fi connection, but it didn't have any effect.

I don't always get immediate notifications because of my Wi-Fi signal. If it's not very strong or at one bar, it won't alert you immediately all of the time.

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