What attack(s) do you have trouble deflecting?

Just curious. What attacks from any boss/enemy do you have trouble deflecting. I’m currently fighting Isshin Ashina, this’ll be my third time beating the game. In his second phase he has a three hit combo. The combo is actually incredibly slow, but I have so much trouble deflecting his first hit. It always hits me, but I can easily recover to block the other two hits. What am I doing wrong here?