Have you ever taken on a writer that had a great first novel but went down hill from there? I'm only curious because I keep reading most authors never sell their first work because they get better at the craft the more they work. I'm just curious if you have ever come across the opposite of that. Thank you for doing this. It's a huge help.
Let me clarify as to how I understood your comment: The author has a first book, but the second tanks, is hard to sell, or we can't sell at all.
We see this all the time with fiction authors. It occurs in film as well, where it's called the "sophomore slump." Directors have been planning a film their whole lives, they make it, it works, and then they have to come up with a film in a much shorter span of time and they don't have it in them.
In fiction it's not always a question of time (since we can't make our authors writer) but ideas. Some people only have one good book in them. It's not a bad thing - most people have no good books in them. They had an idea, a story they wanted to tell (similar to their own or their own with some window dressings) and they told it. Now what?
Generally the author will get a second chance and make it to the third book, unless the second is really terrible. The second-book clause will kick in, and the publishing company will think the second book is worth a chance because it will ride on the name of the author, which is now an established name. It does, to some extent, but people notice it's not quite as good. Then we get the third manuscript in, and it's become even more obvious that the author has really mined his/her brain for ideas and come up dry. They have another story, but it's got very similar themes and the same general message, only it's less interesting. Or it's a departure from form in an attempt to escape that, and it's terrible. We try to revise it, we shop it, and generally advise the author to take the highest amount offered, which is far lower than the previous books. Sometimes we end up selling it to a small press simply because it's been rejected everywhere else, or have an honest session with the author and tell them to try again.
(Note: This does not refer to books that are part of a series. Once a series gets going, the books get bought in bunches or generally just get bought as long as they look basically the same because there's an established storyline and fan base. This has resulted in a lot of bad books being published, but I'm for it because I have a series, and the first book sold. If it does well, the second will be picked up. If I make it to the third, I might be in the clear for all 10, even the ones that are weaker than others)