For example, I Know that part of my calling is to write a book, I have tried to begin writing this book to no avail as the Spirit has yet to lead me to that point, perhaps my understanding is not complete, Perhaps the time is not yet for such a book, perhaps He just needs to finish some work in me that will allow me to write that book? So I wait upon the LORD to direct me and lead me in this regard.
Let me encourage you with something. I write Bible studies. I know it's not the same thing as writing a book, but they used to be very lengthy in fact, and filled with citations, so I'm not talking about three paragraphs and a clever anecdote.
I would often hit the wall as well, but what I learned over time was to wait on God, and I'm not just borrowing from your title. I came to find out usually my understanding of the passage (or Chapter, or sometimes letter as a whole) was not complete yet, and occasionally a good ways off. If I were to have forced the writing of a study before the Holy Spirit was fully able to reveal to me what the passage was actually saying, A. It would have been labor gone to waste, and when you're grinding that sort of thing can go really slow, B. I might have ended up teaching people falsely, and maybe worst of all, C. I would have missed receiving the accurate interpretation of the passage because I moved on from it without taking the time to actually hear from God.
Stick with His timing. It may be hard having to endure it, but "if the Lord does not build the house, they labor in vain who build it."