It has happened to me several times in the past. I will be half way through a book and just lose my way. I had no idea why this was happening. Then I finished one book and maybe I learned something because the next one I also finished. Then I really learned something and realised that I had been an idiot. There was something I didn't understand consciously and then I suddenly did understand it. And it was the answer. And it is this...

You get lost in the middle of the book because you don't know where you are heading. You are heading for the end of the set-up of the problems that the main character must solve, the end of the middle is where the last bit of the problem and the first bit of the solution come together. That is what you are aiming for, and if you are not aiming for that then you are unlikely to hit it.

If you put a lesser climax at that point it helps (that point being the end of the middle of the book). A sub-plot problem, or a secondary characters problem set can be solved or resolved here, and at the same time give the main character something that sets him on the path to the resolution to his own problem set.

I know I will never get lost in the middle of a book again; at least not permanently. Even if I have to go back a bit, pick a secondary character and create a new problem for him and the main character to deal with that then gives a step to the solution for the main story problem.

I will never get lost in the middle of a book again. I just can't tell you how good it feels to say that and know it's true.