Let's look at my manuscript Gorgon-zola!, a manuscript that can seemingly fit on many bookshelves.
First, it's a book about a feisty Italian 30-something woman who finds out that she's the reincarnated Medusa, and you can imagine the boy trouble she has. After all, it's difficult to settle down with a man when every time she gets angry, she's in danger of paralyzing him. She does find Mr. Romeo, in the guise of a reincarnated hero, who just happens to have a fear of snakes. But is it romance? Not unless it's a main component of the story. Whereas my heroine and hero definitely have the hots for each other, unless the story is about my heroine finding true love, then it's not romance.
Second, it's a book with multiple Grecian gods and goddesses playing a starring role. They can disappear and reappear at will, and even summon martinis. So is it fantasy? There's no different world, different culture, and no fantastical creatures.
Third, it's a light summer read, one that I can see a woman taking to the beach for her most personal of relaxation times. She can struggle along with my heroine as she tries to master her powers and she can identify with a woman who sometimes just wants to be left alone. Does that make it chick lit? There's not as much humor as Bridget Jones' Diary.
Fourth, it deals with issues like trust, and solitude, and even briefly touches on religion. After all, it's kind of hard not to when Greek gods and goddesses are forcing you to take sides. It's also for women, so does that make it women's fiction? Women's fiction typically requires greater stoicism, and the tone of my book just isn't ready to go there.
Fifth, it takes place in "real life" and has a magical twist, so does that make it magical realism? Magical realism is often literary fiction with just a dash of "something else" thrown in. Gorgon-zola! is definitely a far cry from literary fiction.
So what's an author to do? Sometimes when you're at your wit's end and you want to call your novel a jack of all trades, you can fall back to those who have come before you. Find a book that you love that reminds you of your own precious novel, and head over to a review site, or read the back of the book. How was this novel described? Another idea is to pick the one that comes the closest and use the tried and true "chick lit with a touch of fantasy". Or ask a critique partner what they think.
And just remember, it's okay to be wrong. It's your compelling query and writing that will capture the eye of an agent and publisher. The market is continuously changing and combining genres in new and interesting ways. Just maybe, the next big genre catch phrase will be that perfect home for the novel you've just written.