There's one question that led to me signing with my agent. I didn't ask it--she did.
"Are you writing anything new?"
I had queried with a project I was really excited about, and was checking in on a full request. My now-agent Jessica responded promptly and kindly (as is her wont) that she hadn't forgotten about it, and, by the way, was I writing anything new?
Fortunately, I was. I had two projects going that I was excited about--both departures for me in terms of sub-genre within young adult, but both really fun writes that I was enjoying. I told her about both, quick pitches that summed up the basics of the plot and what made the stories unique. (Side lesson--be able to sum up your project effectively at any stage in the writing game.)
"I'm so glad I asked!" she replied.
No, I'm so glad I was writing something new!
This led to a request for material, and then The Call, the paperwork, and the All That Jazz, in an admittedly pretty unorthodox situation (yes, I signed with only about a quarter of the manuscript in anything close to polished completion). Unusual? Yes. But here's the thing--many opportunities are unusual.
Which is why it kills me when I see writers putting all their publication eggs in the same manuscript basket. Now, I don't knock taking the time you need to create the best book possible. I don't knock sticking by your project until all avenues have been explored. That's what determination and perseverance are made of. However, when you focus on The One project to the exclusion of others, you might be shooting yourself in the foot and crippling your chances in a competitive market.
You come to a place where you have to decide--start writing something new or revise the old project again? Start writing something new or write a sequel to a book I have yet to place? Start writing something new or spend my time query-query-querying? Sometimes it's not an "or" question, but an "and." (I'm a huge advocate of starting to write something new when you start querying. Keeps your writing mind in a good place.)
In my case--the project I had queried with was a dystopian. I started writing it before Hunger Games came out, believe it or not, but by the time it was finished and polished and entered the query derby, the market was gummy with dystopian and post-apocalyptic. (Cue "Just my luck.") Agents were wary to take them on, because selling was an uphill battle, if the numerous "I really enjoyed this but I don' think I can sell it" rejections were any indication. Yes, I could have trained my focus solely on getting the book out there--querying to fatigue, entering contests, pursuing self-publishing. Nothing wrong with any of these (I admire indie writers incredibly)--but realistically, my goal was and is traditional publication and while bludgeoning my options would serve that One Book, it would not serve my goal. Instead, I started writing something new while maintaining a slow-but-steady query process.
It doesn't really matter why that first book doesn't get picked up--it might be market, it might be timing, it might be that it's (brace yourself) just not as good as your NEXT book will be. Before deciding to pour heart and soul into that One Book, consider writing Something New. This doesn't mean giving up on the One Book--but after all, as a published writer, either traditionally or self-published, you don't get to make a career of one book. Plus there's that whole thing about baskets and eggs.