L. Frank Baum ended up writing thirteen Oz books–he was the J.K. Rowling of his time–The Wonderful Wizard of Oz was insanely popular. The problem with reaching success, you fall into the fallacy of forgetting what got you there.
The Wonderful Wizard of Oz was the best selling children’s book for TWO whole years (1900-1902) as well as a widely popular stage play. Baum probably thought, “Okay, now that I’m known, people will buy whatever I write.” When The Life and Adventures of Santa Claus came out in 1902, it… didn’t do well. People didn’t want just more fantasy, they wanted Oz.
So he had to go back to writing Oz sequels, but he kept declaring, “I’m done with writing Oz!” Then when Queen Zixi of Ix failed to sell copies (even with “Author of the Wizard of Oz” written on the cover), you go back and write more Oz sequels. It made him fabulous well-off… until he dumped a lot of his money into making these new motion pictures and it didn’t do so well. After his death in 1919, other authors wrote an additional 21 books.
The lesson? Don’t forget the gal who brought ya to the dance. Jim Butcher probably really doesn’t enjoy writing the Dresden Files books anymore, but he never disavows writing another, because the Codex Alera ain’t gonna pay the bills. He himself indicates that he loves sword and sorcery fantasy the most… but that subgenre was last in vogue in the 1950’s.
Of course, it’s good to stretch out, because you never know what might take off. One of my favorite authors, David Weber, started a different series on the planet of Safehold which can only be described as… um, Reformation Wars with a little sci-fi. I enjoyed it… mostly. There was one book where he deliberately avoided the wargasm which it had been building up for, but he paid it off in the next one.
What other examples can you think of? Authors who obviously didn’t like what they were writing, but kept cranking out sequels nonetheless. I’m sure I could think of some film examples, especially with actors. Let me know in the comments below!
Albigensia Press 
That us the trouble with our world and our economy. Having to sell to survive. Miserable and degrading.