I realized that my brain had been in editing mode. It was able to chop scenes, cut words, or reorder sentence structure, but creation of something new, of that new scene that I need to add, was entirely a different muscle. They must be different parts of the brain. Switching gears was something I hadn't counted on needing to do.
I ended up revising everything in that chapter until I got to the part where I needed to add the new scene. Then I tried to switch gears and just focus on writing, not going back to editing for the rest of the day.