At least as far as I am concerned, the biggest hurdle in the way of writing a longform story is that I tend to love beginnings. So by the time I am in the middle, I start looking to see if the end is in sight. When it looks far away, I lose interest.
Of course, this is also why I love doing short stories. The beginnings are fun and the end always within sight. Plus, I love character-building. I envy those who can write novels without sweating buckets.