That happens to me sometimes. Some stories flow with ease and others don't work out as well or take more time in order to get work done. I've had a couple of really great years where I managed to write so many words in November. I've had other times where I've struggled to not even reach goal. I even have a novel I have been trying to finish for years but still have like 5 chapters to go. Never know how it will develop until I start writing.