I think I simply forgot how I write; I need to throw down the story as roughly as possible, come back to it and start seeing how it needs to work, then come back to that again. Somewhere in all that the magic seems to happen, and I end up with a finished piece of writing.
An interesting observation is that I find myself writing in discretely connected sections of around 800 to 1000 words. I don't know if that is because of the way I've been trained, or if that is my natural style, for this book anyway. I will just have to see how it works out.