My best guess, regarding your writer's block, is that on some level you feel you're missing pieces, or events are not taking the right direction. From a logical, conscious stand point the events may seem to follow the right path, but our subconscious has a habit of telling us things in conspicuously subtle ways, like an aversion to writing what we most want to write. My first thought, therefore, would be to assess where things are going and play around with different possibilities and outcomes, or different ways of getting past book five and into the next leg of the journey. My second thought is for you to step away from the project completely to work on something entirely unrelated. By occupying your mind with something unrelated, you allow it to keep gnawing on the problem in the background without trying to force the resolution. It also allows you to feed that desire to write without the same pressure or expectations you put on your approach to this series.
Anyone who stumbles on my answers will see me say it a lot, but I find that role playing with other writers helps a lot as well. Explore characters and plot ideas without the pressure of turning it into anything specific, get instant feedback on what could use improving and how, and write something unrelated to your main projects to detangle your thoughts and unwind emotionally. The Innovator's Last Stand is where I go for this very purpose. It keeps me in the writing frame of mind, but always offers something new, so I can't suffer from too much exposure to the same plot with its same problems.
If the blockage problem is not enough detail somewhere, then that's really what the focus needs to become. I found a massive aversion to trying to write a recent horror piece when sitting down and "letting it flow" was no longer working. I'd hit a point in the narrative where I really needed to know where I was going in order to get there. The problem was I had no idea where I was going. I spent weeks (I know it's not a year, but blockage is blockage) wanting to write, but hating the thought of trying to write it, and it wasn't until I was relaxing in a bath that I finally let my mind wander without demanding anything from it. Playing with where I left off, I managed to come up with the rest of the story straight through to the end. With an example more like your own problem, a series I have always running in the background discourages me from writing the actual prose because there are such huge connective sections missing from the plot connecting the books. It's like this hazy blotch of "and then . . . happens" between "here are the scenes I know!"
For me it's less like I want to explore those hazy patches and discover things as I reach them (then it's like a surprise!), and more like I want a clear idea of the major points in play before I try describing them to the reader. That said, when you think of a concept as being inflexible, you can plot yourself right into a corner, the kind of corner that sucks all the "want" out of writing. If you look at future events as being a little more flexible to those you establish first, you may find yourself less reluctant to complete the journey.
Also, there's software for the first point Jonathan suggests. I use yWriter and Keynotes NF, personally. Keynotes lets you make your own categories and tabs, which is great for compiling world building information. yWriter actually has sections built into the program for characters (major, minor), locations, items, notes and scenes. Inputting the number of chapters you wants starts your project, and you can write in as many scenes as you like, which are modular and can be moved around between chapters. It also tracks when a character is a POV character, how many scenes they're in, and how many words are involved in those scenes. It even lets you record your goals, conflicts, and outcomes with each scene. Its word processing capabilities are quite limited, though, so I find it best to write my scenes in the word processor I prefer, and port them to yWriter for my records. You can keep track of which draft you're using this way, too. All in all, I find it keeps the clutter down in my writing folders.