That being said, Kotone/Shinohara's NTR won't begin until Kotone/Koide's NTR has already advanced, and IIRC this happens when you missed at least 2 feast prep session with Kotone, usually happening during Night 2 at salon with a (?) indicator. I think it would be better that you provide a save or a screenshot of your progress for us to look at this further, or load a previous save/start new if you've missed the flags.
I can't publish a print, as I already saved it above.
But thanks to your explanation, I think that's exactly what happened.
The game is already starting to build Koide's relationship with Kotone, but there has been no significant progress.
However, regarding the "?" symbol, I skip some depending on the action that takes place, so I go back with the save and do another action, since the next day the "?" Come back at the same time to continue the story.
There will probably be an option "?" in the salon for kotone, and i did another action with another girl. Even though the game warned that if I didn't compare Shinoraha it would prepare for the "feast", I didn't realize that I had to do it every night.
As the game doesn't say which day the kotone will go to the "banquet", I took this ntr as a mistake.
This is a really good game. As I've read, you should play this game more than once to get other scenes on other routes, or, in this case, avoid a route.