I was thinking about long running series where the author has run out of ideas and the book just becomes a replay of shtick and cliches.
That because I recently read or tried to:
1. The last Spenser novel by Parker and the new one by Atkins. It was all quips and no story. Spenser is tough. Susan is beautiful - but they are both 85. Yawn.
2. J. Jance - Brady series. Ok, you have a crappy mom. You have a family - so what.
3. Anita Blake - what more can be said. All plot disappear for endless babble about Anita's orificies and who is a gay vampire. Yuk.
The Bosch books (except for the Hong Kong one), Daveport and Flowers, James Lee Burke - manage to keep some interest. So does Gabriel Allon. While they have backstories - it doesn't overwhelm and there is a plot.
SM Stirling Fire series - how many more books till the reveal - who cares about folks who think they are elves. The Island series was tightly plotted for 3 books. These books - endless. I understand the need to make a buck - but ...
A couple of David Eddings's series, most specifically I recall "The Tamuli", his sequel trilogy to "The Elenium". The first series I enjoyed, but in the second series all sense of threat to the characters was replaced with quippy lines and god-like (in a couple characters' cases, literally) competence in the face of all their enemies.
"Outside of a dog, a book is a man's best friend. Inside a dog, it's too dark to read"- Groucho Marx