I've had this question since my first session with the game and I've never found an answer to it: If a survivor is already knocked down, can they also be knocked back?
Logically, it seems like a person lying on the ground would not be affected by something such as a lion's roar that knocks them back. But the rules don't technically say that knocked down survivors are immune to knock back. Or did I just miss that somewhere?
One of the main ways to get knocked back is for something to occupy the same space you do. If being knocked down make you immune to being knocked back the logical alternating would be being crushed to death instead.
Another way to look at it for the lion example is that when you are knocked down you can still dodge. You basically rolling around on the ground not laying there limp. The same would be true for the lion roar, just being on the ground doesn't mean you aren't frantically trying to scramble away.