Every visual cue hints at Nanami missing his Ratio on Dagon
This is something I wasn't very big on or even fully convinced of myself at first, but as time has passed I have grown to consider it among the most important aspects of Nanami scaling. Ratio has 1) Always been visually displayed to us when it's been activated, even if characters in-verse can't see it and 2) Stated to be something Nanami needs to create and aim on a target, so it's absolutely reasonable for us to assume that not only is it not an option in every hit, but it can also be missed where it is. Most of the argumentation for this can be found in the first image. While I'm not a fan of anime scaling as it often introduces things which weren't in the manga, I have no issue with using it to clear up things which were also present in the source material, and I think this is a great example of it as in the anime there is no Ratio axis whatsoever and the sound it makes as it's skidding on Dagon's forearm is very different from the sound that comes with almost every other Ratio.
Where that Ratio axis was aimed is where opinions among those who believe that it was missed start to diverge, though. I don't have a strong opinion on it either way personally. The most common opinion seems to be that it was aiming for Dagon's head, but he was blocked by the forearm halfway through. The major dissenting opinion is that it was aimed for the forearm but activated too late since the block came last second and missed. Either way, an impromptu block has some interesting implications, because wherever the Ratio was drawn, Nanami didn't get there with attacking Dagon's forearm in mind, so his swing was stopped in motion. Stopping an accelerated attack before it's allowed to reach the point the attacker is projecting to reach greatly reduces its acceleration and impact. This is applicable in real life too if you want to look it up, a more refined/defensive version of it referred to as jamming/wedging is found in boxing. The head argument allows for more lost force (higher cap on a full blown swing) due to also introducing the element of surprise/shock to being caught halfway, but I don't think it makes a big difference either way. I leave the rest of that to you.