I've been writing (and selling) for a very long time now, but still only about one story in ten sells. I keep my stories out there. As soon as a story comes back from one editor, out it goes to another. But I've found that if a story doesn't sell to a top market: Analog or F&SF say, then it doesn't sell to the less well-known professional markets, either. And yet, looking over the stories, I am unable to tell why one story sells and another doesn't. I wonder if other writers have had similar experiences.