Stephen King - After the awful Cell I finally gave up ion him altogether.
Dean Koontz wore off pretty quickly. Someone needs to tell him it IS possible to have a book without a special needs child and a golden retriever.
Patricia Cornwell can fall into a hole.
Clive Cussler. The earlier novels were hamfisted but fun; as the series went on its endless way the absurdities just piled up and piled up. I think the final straw was when he revealed that the long-haired hippy-dippy computer guru was REALLY a Young Republican all along, just wearing a disguise. Or when he had his hero fly around in a Moller AirCar (so, Clive, how much did Paul clip you for?)
But then, the cheese was always there. Fans will remember that Dirk Pitt - sorry, make that Dirk Pitt™ (seriously) - responded to the tender young thing who hadn’t been laid in seven or eight years because of an abusive lover by… raping her. And she thanked him for it.
Cussler just got more and more full of himself, and it showed in every book. Enough, me finally says, and dumped my entire collection on the local UBS.
John le Carré. His first two novels were simple detective stories (though Smiley was in each) and outstanding. His early spy novels were also great. But at some point he decided to turn his books into sort of diatribes against Anglo-American imperialism rather than fun stories.
I’ll still read the first chapter of his latest novels: his specialty is the vignette! Indeed Secret Pilgrim, a collection of shorts, might be my favorite of his books.
I loved Kurt Vonnegut circa 1970 but got tired of him ca 1973. But that’s a long time ago; maybe I should try him again! 
Stephen King for me too. Sometimes it’s his overly long description of things that aren’t too related to the story; sometimes it’s the tangents he goes on that don’t have anything to do with the story, and finally he just seems boring now.
Spider Robinson. He was great back when he was just getting started, but once he got some recognition and awards and GOH status, he decided he was the next incarnation of Robert Heinlein & John MacDonald rolled into one, and, really, he isn’t.
Vernor Vinge. LOVED his early stuff, but Rainbow’s End did not work for me, and I’ve had trouble getting into Children of the Sky.
I hope he can return to his earlier form.
Piers Anthony and Orson Scott Card because they’re gross creepers.
Jean Auel because she forgot how to write after the first couple books, but refuses to stop writing.
Jean Auel
John Grisham
Robert Ludlum
Tom Clancy is still on my good side, but barely. His newer ghost-written stuff really suffers.
Piers Anthony, for sure.
Stephen King went in a wave: I used to love him, then for awhile he was writing nothing but crap, and now I kind of love him again.
Scott Adams: Used to love “Dilbert” until I read his horrible misogynistic rants on his blogs. I still chuckle occasionally at a strip but I no longer buy his books.
He doesn’t write much anymore, but I used to love Dave Barry but in his later stuff he became essentially a parody of himself. I’m glad he saw it was time to mostly hang it up.
That’s who I was coming to say. I enjoyed his early books, but after the second or third time Cussler inserted himself into the story I’d had enough.
Tom Clancy, for sure.
James Lee Burke: I really liked most of his novels, but the most recent Dave Robicheaux book was pure crap, and I don’t mean that in a good way. It was bad enough that I posted a review of it on Amazon.
Before reading 11/22/63, I would have responded with Stephen King, and I think a lot of people make it an automatic response nowadays. But that was a decent book and a good read.
A few that have already been mentioned - I changed for JRR Tolkein exactly the way **Ulfreida **described.
Quite a few where the author changed; Orson Scott Card (religion), Stephen King (drugs), etc
And a few where they didn’t change enough, Piers Anthony seemed to write the same book over and over.
One that I don’t think has been mentioned is Anne McCaffrey. I lost interest. Way before co-authors were being brought on board. Six Pern books were enough. The other series she wrote also suffered from multi-generational soap-opera-itis.
I have to agree. Also, once I see someone has taken on cowriters, I’m done.
Douglas Coupland
I thought Gen X was brilliant, I adored Microserfs. I tried Generation Y this Summer on a flight and it kept me occupied and the time flew- but MAN was that annoying. All that non stop cleverness. All that wit.
With great, great reluctance, I think I have to add Tad Williams. 
Otherland is one of the greatest series I’ve read. But I keep trying Shadowmarch and I keep dropping it. Everything I read about it makes me think I should love it but I just can’t get into it.
Try his new series, which began with his new book, Streets of Heaven.
I had more trouble with Otherland than any of his other series, but still liked it a lot.
Piers Anthony - first it was a book that got published because he was popular (Mercycle). I stopped buying his books after that, but I held on to the ones I had already bought. Then I decided to re-read the Adept series, and at the dénouement of one of the books, he basically had the hero threaten to rape another character.
A couple of romance authors
Catherine Coulter - good writer, good plots, but after a couple of books I realized that she had a rape in ever single book. On screen so to speak.
Mary Balogh - another author who was good at plots, but after a few books, I found her style really weird and remote. Remote is bad in romances.
David Eddings. After writing the same book five times in a row, it got a little tired.
Anne Rice. Don’t like her new stuff and the old stuff wasn’t nearly as engaging as I recalled.