Welp... you can't just spam H-buttons (A/S/D/F) - there are certain animation timings when you need to press H-button (A/S/D/F) several times. I can't say exactly when because the animation speed is random there... When the H-animation starts then you have to press certain button (A/S/D/F) several times.
What more can I say?
Well it’s like... I don’t know... If I try to tell you how to parry in Dark Souls;)
You also need to press a certain button at the moment of impact;)