Often writers churn out basically the same story in series, just replacing the names of the main characters and altering situations slightly, but nonetheless you know what's going to happen. I think romance writers are the greatest offenders. I used to love reading Christine Feehan's Carpathian series, giving the first book that I'd read of hers 5 stars, then fervently snapping up more, but eventually her repetitiveness with giving me basically the same story as well as repeating annoying sayings, like "molten lava" made me lose complete interest in her writing. I remember the last book I bought of hers I got barely a chapter in and gave up.
So, what makes writers repeat the same stories varying to a certain degree so their readers think they've bought a new book?
1) The writer has a formula that they know works, and they don't want to risk writing anything different. Yes, they will lose some readers, but by then we've already bought quite a lot of their books and they've made their profit, so some losses aren't too damaging.
2) They are a write by numbers type of author, they're skill not progressing. They don't know how to write different stories. Full stop. There is no originality there.
3) Their publishers know that this particular formula sells and they don't want to veer away from it, because of course it's proven, therefore it's money in the bank. Hence, they ask the author for more of the same.
Personally, I hate it when authors write formulaic books, which is why I don't do this in my own writing, my books in the same series quite different. For example "Behind the Hood" is a totally different kettle of fish to "Behind the Tears" while "Graffiti Heaven" (the first in a prequel series) is different again. Yes, there are similarities in the sense of my books having harsh and dark elements, but that's it, just a stylistic, genre thing. Instead, the story progresses, changes, like life does. "Behind the Hood" was more nuts, a crazy bunch of teenagers raging out of control, starting from one bad decision that snowballs into a fall-out avalanche of disaster. Whereas "Behind the Tears" has more of dark romance elements to it, where the characters are coping with their difficult relationships. It also has a strong family aspect to it, because unlike the first book, this tale is about three brothers instead of a selfish gang-leader who wrecks havoc on other people's lives.
But, to be fair, my books aren't romance books. Although some of them may have dominant romantic strands, it's just a part of life. Instead, I prefer to write realistic fiction, my genre probably being relative to a program like "Sons of Anarchy", where you get romance, but it's only one aspect of life, the other main issues also just as important: business dealings, family issues, etc. And like SOA, you also get the criminal elements in my writing, because it's dealing with a certain section of society. I must say, I'm a huge fan of "Sons of Anarchy". Before last year I hadn't heard about it, and was surprised that it had been going since 2008. So, when I got back to New Zealand this year, and since it's a genre I like, I went and rented it from Blockbuster, and now I'm up to Season 3 and am totally addicted to it, the story-lines so well done. Of course there is a bit of jumping about with the timeline, but the writers have done such a great job with making the transitions as smooth as possible. Plus, the story-lines are not always predictable, which is a good thing and is something hard to find on television, and is why I prefer not to watch TV or films now, because I basically know what's going to happen. But "Sons of Anarchy" doesn't have any problems keeping my interest, and the story can totally surprise me as well as kept me pestering my husband to watch more. Three episodes a night, no problem :) I will try to get a season a week if he let me.
Again, I suppose it's not fair comparing a romance writer to a television program, but there are romance writers out there who know how to be original and progress a story, do world building. J.R. Ward writes the same genre as Christine Feehan, and she progresses her stories. I really like her books and can't wait for the next one (really can't wait!) because she doesn't just regurgitate what I've read before, she gives her readers new elements to her stories, gives us something interesting. Of course, she has couples getting together, which are givens, but she doesn't have stereotypical characters like Feehan does, Qhuinn and Blay's coming story proof.
So, what I'm saying is: I HATE FORMULAS. Be original in your writing, create a world that a reader can believe in, even if it's filled with vampires or bikers, those aspects are irrelevant, just as long as you capture your reader. Also, progress with your writing / the series. Give the reader something new. Don't continue to create write-by-numbers books. Doing this is okay for your first couple of books as you're cutting your teeth as a writer, but definitely don't keep doing it, and most certainly not on your tenth book, etc, like Christine Feehan does.