"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."
Every writer knows that sometimes you have to put an idea or a partial WIP aside and let it sit for a while. Even though you might want to write the book, the time just isn't right. When a book is meant to be written, the ideas flow. The actual writing may still be hard at times, but the author knows that they can get through that if it's time for the story to be told.
Sometimes the best thing you can do for a book is to step aside for a while and let it percolate a bit longer. :-)