Not full screen for some of my apps

So, I’ve used some apps and I’ve noticed for about a while now, ever since I got my new phone that the app isn’t fully full screen. I have done the zoom trick but it still doesn’t work. It’s been getting on my nerves and I haven’t found anything else for help. Does anyone have any ideas/tips? Or have anyone else gone through this and find out the way to fix this that isn’t the zoom hack?