Edit: I tried with a 2nd controller but the stamina restores too quick and it's kinda wonky and too difficult to test by myself.
I think I just accidentally stumbled onto something important, it seems how easily you get knocked out and how long you get knocked out for is tied to how much stamina you have left. That's probably why sometimes it seems like you hit a player 10 times and they don't get knocked out, because their stamina is full. Alternatively if you get knocked out too easily one hitter quitter it may mean it's because you're always running your stamina too low.
I'm not positive if this correct. Can anyone else confirm this?