Stories are tough. It's not that writing a short story is harder than writing a novel, but writing a book of short stories is really difficult—for me, at least. My last collection, Blue Spruce, represented seven years of work. A book of stories is all beginnings and endings, each of which has to be just right. Whereas a novel is mostly middle. If you get stuck while writing a story, then you're just stuck. But in a novel you can go and work on a different place.
[Note: For those participating in Carl V.’s Autumn Reading Challenge, David Long’s new novel, The Inhabited World, sounds like a good prospect—a most unconventional ghost story. I haven’t read it yet, but I’m sufficiently intrigued by what I’ve read about it to add it to my list.]