Have been able to perform master strokes since the patch, but it’s still a bit rare. Part of the issue people are having is that a lot of your capacity in doing this appears to be related to framerate. After I disabled V-Sync, thereby going over 30 FPS, I was able to get the timing right. Before the patch I was unable to perform a riposte, outside some very rare and lucky occasions where I anticipated the move well before they even raised their arm.
Frustrating, because you really only have a quarter second to press the button in order for it to work. If I were Warhorse I’d make the window for it longer, and simply require that you both guard and attack at the same time for it to fire off (as bernard implies you need to do). It should not be as machine sensitive as it is.