P20 pro doesn't ring if I'm using an app

[Android Central Question]

Previously if I was (for example) playing a game or browsing the internet and someone called me, my phone would ring. Just lately, my phone no longer rings, but a small notification pops up on the screen. If I have been playing a game and just put my phone down, without exiting the game, then my phone doesn't ring and obviously I don't see the notification.
This applies to phone calls, messenger/text messages etc.
It's quite frustrating and I can't find how to make the notifications priority over the apps