Oda changed King's powers after he started to fight Zoro. Oda increased King's defense and durability. Lets look at facts to back it up.
Zoro's ogre slash cut King open and made him bleed. King had his flames active.
Yet, Zoro's death lion song against King didn't make him bleed. That was one of his strongest attacks and a stronger attack than ogre slash. That is the attack Oda consistently has Zoro use against characters who are hard to damage because of their durability/toughness/defense had no effect on King. It couldn't make him bleed even though it split open his face.
Zoro, who can cut and scar Kaido and scare Big Mom with his AP couldn't do anything to King after their battle started.
If Marco kick make King bleed then why didn't King head being split open make him bleed? The answer is Oda changed King's durability/defense.
Like I said. King is harder to damage than Kaido or Big Mom. Damaging them doesn't automatically mean you can damage King.