I think one of the problems with writing the middle of a novel is that the beginning and end come in so clear (for me anyway) - almost like you're watching them on TV - while the middle doesn't. The middle requires more thought to know where the action is headed and what events must occur in what order to get to the end. And that requires sitting down and actually plotting the novel.
Now, I tend to do this before I start writing, put down a tentative outline of what needs to happen and when. I don't put in too much detail, because the outline tends to change with the writing anyway, as I get new ideas (generally better ideas) and become aware of inconsistencies and problems that need to be worked out. Which requres a lot of sitting down, after the book has been started, and organizing my thoughts and ideas so that I can keep writing. The problem is, that thinking and planning time doesn't feel like work because you can't see WRITTEN PROGRESS. And yet, without careful planning and thought you get a worse book (just stuff happening that isn't necessarily requred to tell the story you set out to tell - you know, like that scene in the movie that LOOKS cool but makes you ask, "Is there a point to all this?").
So I've had to revise my writing schedule to allow for planning time. That is, time when I'm in my comfy chair with a pad of paper and a pen, jotting down ideas and hashing out the problems rather than at my computer trying to write a scene that just isn't coming (because I haven't decided where it needs to go).