I'll post something that will explain how to do it later tonight. I don't have access to the game till then.
Edit: Make sure you are hitting the melee button.
Also try this : "If you notice your system slowing visibly during QTEs, or your button presses seem to have no effect at all, you may want to turn down your graphics settings temporarily until you pass the QTE. Some players have reported that simply turning off the "Tesselation" option under Options > Graphics > Advanced fixed their QTE issues."
Graphis in this game at too high a setting screw up the QTEs.
If nuking your graphics doesn't work, then you either need to hit it earlier or faster.
I can't be of much help other than this until I can get at the actual game, so I hope it helps you.