Marco & Vista for example failed to cut Akainu by forcing his body to solidity, therefore, their Armament Haki is not strong enough to hack through Akainu's DF because it has a greater level of resistance to Armament Haki and conversely, their Haki isn't strong enough to fully treat his body like solid.
^ Exhibit: Chapter 574.
Pay attention to the "
WHAM" SFX, it indicates impact or blows that landed against a surface as "
these examples" show. This is to repudiate the idea that Akainu created a gap for the attacks to miss him. Attacks that do not land on anything but air don't show up as "
WHAM", but
"WHOOM" or "WHOOSH" that indicate cutting through air at high speeds.
Whitebeard on the other hand had no issues hurt Akainu, why? because Whitebeard's Haki is at the very highest level, the Magu Magu no Mi's level of resistance is exceeded by Whitebeard's Armament's strength, consequently, he's able to treat his body as a solid/tangible object.
^ Exhibition: Chapter 575.