I know it's a great tool against ganking, but as it is today, it doesn't work as intended and that's pretty obvious.
Sometimes it charges with one strike, sometimes it takes 3 parries to do the trick, sometimes it doesn't even charges at all.
It's utterly broken and makes the game unfair because it seems like mere luck if you get revenge or not.
Your tag system doesn't work properly, fix it and, in training mode, put an image explaining clearly what charges revenge and how much an action (heavy, light, bash, GB) will fill the enemy's revenge meter.
That should be shown to every player to know.
And that's just one of the mechanics that's broken within the game, it really looks like you don't have a clue on how to make a game that actually works.
I still play this because there's no game like it in the entire market.