When reviving allies it seems that the button isnt reacting unless you stand in the perfect spot.
Sometimes you stand by your downed friend but the prompt isn't showing so you can't revive unless you are in the perfect spot.
You could adopt the revive system from "For Honor". There you just need to be within the right area of a friend and you can press the revive button even if the prompt is showing.
So on xbox, I would press X to revive. In for honor you could start rhe revive as long as your character is within reach but while the running animation haven't finished yet.
It would help alot to earn to extra micro seconds in a boss fight, just finding that sweet spot when you are actually able to revive