"It is worth mentioning, for future reference, that the creative power which bubbles so pleasantly in beginning a new book quiets down after a time, and one goes on more steadily. Doubts creep in. Then one becomes resigned. Determination not to give in, and the sense of an impending shape keep one at it more than anything."
Of course, this is not a hard and fast rule. I've had stories fight and struggle to be told from start to finish and others flow like a river in a rainstorm, but it does seem to be the case more often than not, at least for me.