The post above me is correct. Some notifications are meant to disappear completely if they are viewed elsewhere - this is something any app that issues notifications can do.
The other thing you might check is related to a new aspect of the way notifications are structured in iOS 11. At any time, if you look at the notifications on the lock screen, the ones immediately visible are those that have been delivered since the last time you unlocked the device. After you unlock and go back to the lock screen those will be relegated offscreen to the "Earlier Today" section, which you can access by swiping up on the lock screen. In the notification panel you see when the phone is unlocked and you swipe down these "Recents" are always immediately visible and the "Earlier Today" section is contiguous, rather than having to be swiped onto the screen.
Does that make sense? What I'm really saying is swipe up on the lock screen and see if your missing notifications are there. If not, they're probably being cleared as
@Nekomichi described they sometimes work.