Though I enjoy writing short stories, both to play with concepts that would be unsustainable in long form and for their own sake, I'm a novelist at heart. I also love stories (short and long) that aren't confined by their written dimensions. The characters had lives before the story began, and the resolution to the plot problem is often, "Yes, but ..." So it's probably not surprising that many of my rejections include the sentiment that "this should be a novel," or "this reads like the first chapter of a novel." I also get this from readers and critique partners, or the more positive, "I'd love to read what happens next!"
I ... don't know what happens next. That's it. That's all she wrote (literally).
And sometimes, it puzzles me. I wrote the concept as a short story, and to me, that's (usually) as much potential as it has. The plot dimensions implied after the end of the tale aren't intriguing enough to grab my attention. The world as constructed doesn't have enough complexity and interest to serve as a framework for a novel. Sometimes, the characters aren't people I want to spend that much time with. So I wonder what everyone else is seeing that I'm not. (Sometimes, as in the case of Scylla and Charybdis, it just takes a few years of incubation.)
Maybe I just expect too much from my novel concepts; maybe it takes too much to grab my attention, when successful storylines have been spun from much less. In some cases, I think it's because my intimations of past and present aren't done correctly: they carry too much weight, raise too much curiosity. In others, I think I've got just the right blend of sleight of hand to suggest an entire world behind a paper diorama.
Or, since I'm an incubator and I do most of my story development on the backburner or in my subconscious, maybe it really does take years, and I'm still waiting for some of those stories to burst forth into madcap sagas.