Hi guys, so I'll just put my issue here.
I am using a Huawei P10 phone, same as my best friend. Version 8.0.0.
He gets all notifications through normally, even if phone screen is off for X amount of time.
Where as I have problems with Instagram and Google Hangouts.
I only get instagram direct messages through, if I go directly to the Instagram app.
I still get the android notification of a message i already read, over 1 hour later or so, which is weird.
For google hangouts, i don't get any notification untill i manually enter the app.
Gmail, SMS, and everything else works perfectly fine..
Since my friend is in the US now, i cant compare his settings to mine...
EDIT:
So what i did, and it instantly helped me, on my Huawei, I went:
settings>>battery>>Launch>>i checked instagram and hangouts manually, and made them run in the background.
This fixed my problem, as others said, it was Google's hardcore battery saving option, it kills some apps in the background.