[fjtorres]

For me, it's really all of the above, on a case by case basis.
Some series, the author loses sight of what made the books interesting for me or starts focusing on what they think their audience wants or they start believing their own PR, or worst of all, starts rehashing the same old same old.
Some authors I'll stick with even after their work has devolved into formula writing, others I'll just stop cold turkey without a twinge of regret.
Some that I've stuck with have rebounded.
(Xanth comes to mind; I loved the original trilogy, was less than thrilled with some of the latter volumes---*cough*NIGHT MARE--- but stuck through for a while and enjoyed the Dolph and Ivy arc, especially MAN FROM MUNDANIA. Shortly after that I just stopped reading, as the "adult-friendly" aspects of the series got overwhelmed by the fan-friendly stuff. I'm glad it's done well for him but I don't think his target audience includes me anymore.)
Others...

Bottom line: when I enjoy a book, the author earns a bit of "credit" with me that he adds to or draws upon with follow-up efforts (series or not) depending on how he delivers. But if the account runs dry, I'm gone. I might return if a *lot* of reviews make me reconsider but it'll take time.
And it's never quite the same afterwards.
(I never did quite "forgive" Heinlein for NUMBER OF THE BEAST.)