"There are some books that refuse to be written. They stand their ground year after year and will not be persuaded.
"It isn't because the book is not there and worth being written -- it is only because the right form of the story does not present itself.
"There is only one right form for a story and if you fail to find that form the story will not tell itself."
~ Mark Twain
See also Writing Do's and Don't's. You'll find more about this and other causes of writer's block in The Voice of the Muse: Answering the Call To Write
Thanks to Creative Screenwriting's CS Weekly newsletter for the quote.