Squandering the Reader's Trust
I fell in love with the TV show HEROES during the first season. It was different and clever. The writing was smart, the worldbuilding interesting, and the plot worked.
Rarely did the series fail on most levels through that first season. The fans were fierce, the buzz was good, and HEROES appeared to be a major hit.
Then the second season came. The plots went nowhere, many of the characters we cared about were tossed aside for new annoying characters, the worldbuilding faltered, the whole series went to heck, and many of the watchers went elsewhere.
I only stayed around because, from a writer's point of view, it was a bad accident I couldn't take my eyes off. My weekly show autopsy was a class on how lazy writing and a smug certainty of keeping the fans no matter what could destroy a good show.
When the third season started, I watched, but three was a regurgitation of the first season with exploding cities, premonitions, and Sylar and the other characters returning to their old ways.
I also realized that I no longer cared about the show anymore, and I stopped watching.
As I was thinking about my reaction to the show, I realized that the first season taught me to trust the writers to give me the kind of show I'd enjoy, and the second season squandered that trust until little remained.
Novel writers can do that, too. Each book builds trust between the reader and the writer, and the writer has to be faithful to that trust for the reader to stay.
Common ways to betray that trust are writing by rote with few surprises, worldbuilding changes for the writer's convenience, and simple boredom on the writer's part which most readers can sense.
That series you are writing may be a major success, but unless you are willing to keep stretching yourself and to keep pouring your creative energy into it, you are better off starting something new before all your readers go away.