Insert into Post button is missing for certain images

I thought I was crazy, but on some of the images in my media gallery, there is no “insert into post” button. see the picture:

this is only for some images though, and the button is there, like it’s supposed to be, for the others.

what could be causing this to afflict particular images and not all of them?

If you have a post type that doesn’t have the editor, uploading media in that post type will cause it to not have an “Insert into Post” button. Not sure if that’s your situation, but personal experience says that could be the reason 🙂

I would definitely disable all plugins and switch to a default theme first to see if one of those is causing a conflict. Absolutely do not modify the core.