There are already questions about this, but they are quite old and the answers didn't work for me, so I think there should be a separate question for newer Android versions. Also in my case it's intermittent.
Gmail notifications do not arrive most of the time, although very few times they do, for example after a reboot. I have updated the Gmail app, I would also update Google Play Services but it doesn't show up as an app so I guess it's integrated and updates automatically. I have also cleared the Gmail app cache data.
The notification options are all activated for this app (I have checked in the system's notification options and also in the Google options)
Any ideas for something else that I could try?
@Andrew .T's comment seems to be the key. I wasn't able to stop the system from killing my apps, so I installed another email app with Gmail support. Then that app showed the notifications, other times both of them show notifications, and now funnily enough Gmail is showing notifications and the new app is not :/