Basically, your follower cannot revive you because they cannot pathfind to you. even if they are standing right in front of you. if they cannot "see a path to you" they won't initiate revival. I was nearly to the top of a tower when i suffered a fall, because of where I landed, my npc follower couldn't pathfind to me, so i just watched her walk back and forth in front of me while I wrote this.
