Romance books count for about half the genre books sales at Amazon and other online publishers. Romance fans are fiercely loyal and they know exactly what they want: predictability. They want to know that the two main characters are going to meet, will come across some adversity, and will end up Happily Ever After at the end of the book (or at least Happy for Now). There's an infinite variety of ways that you, as a romance author, can hit those high points in the story, but you'd better hit them if you want to hook your readers. These touch points, or tropes, actually make up a sort of secret roadmap that shows exactly how to write romance books that sell, over and over again. The trick is figuring out the map.
Art vs. Money
If you're looking to write the book of your heart, the tale that's been sitting inside your brain for years, you may look at this process in horror. There is no magical muse involved here. It's pure science, and it works. If you're in the business of writing books for money and you want to do it over and over again, this is a proven way to do it. One type of author isn't better than the other -- there are just two types of reasons behind sitting down at the computer each day. If you want to be a professional, you'll take advantage of all the tools you can find.
Tropes and Maps
Romance books are remarkably alike in many ways, and for a good reason. Authors have found that, during the length of a romance novel, most people expect certain things to happen in a certain order. The secret is that each separate romance genre has its own rules. Billionaire romance has one set, which is very far from shifter romance's rules, and neither has anything to do with sweet western romance books. Each successful book within a genre will follow the rule book, but it's like learning the rules of basketball. They may be rules for a game with a ball, but they won't do you any good if you want to play baseball. The first step in building your successful map is to pick your genre; decide what kind of romance author you want to be.
Once you've settled on a niche, get 20-25 of the best-selling examples and read them. Read each book twice. The first time, read for enjoyment, paying attention to the rhythm and flavor of the story. The second time you read each book, write a short outline. Make a short note about each chapter of the book. In chapter one, what happened? What were the actions in chapter two? Do this with each chapter in every book. This may seem like an enormous amount of nitpicking work, but you'll begin to see a definite pattern in each set of books you pick apart. Maybe the two main characters always meet with some fireworks in the first chapter. Maybe there's an alternating point of view, switching at the beginning of each new chapter. Whatever it is, that's what works the best for readers of your niche.
Write a chapter by chapter outline, including all the information you learned. Make lists of the most common types of main characters, jobs they're likely to have, secondary characters, and common reasons for discord between the characters. Use this master outline to plot and create your books, one after the other, and follow all the tropes each time. You'll be giving your readers exactly what they want, and the increase in your book sales will prove it.
Keep the Faith and May the Force be with You!