I've seen novels that have grown out of one story in a collection. But it hasn't occurred to me to take any of those stories and build on them. They seem very finished for me, so I don't feel like going back and dredging them up.
“I've written 16 children's books and five unpublished novels. Some of the latter were breathtakingly bad.”
“I have more freedom when I write fiction, but my memoirs have had a much stronger impact on my readers. Somehow the 'message,' even if I am not even aware that there is one, is conveyed better in this form.”