I was listening to an interview with Margaret Weis and Tracy Hickman from the recent Gleemax podcast and I remembered this phenomenon with some authors. Writers like Margaret Weis and Terry Goodkind are examples of authors whom fans extremely like the first few works (Dragonlance Chronicles in the case of the former and Wizard's First Rule in the case of the latter) but not so much in their subsequent works. Don't get me wrong, these are good writers (although I wouldn't say great) but somehow, their other works just don't have that "spark" that was present in their first novel/trilogy.
In the case of Weis, I don't know why. I read her Deathgate cycle and while it was an okay series, it didn't seem that impressive to me. Returning to Dragonlance in the War of the Souls trilogy, I think the story suffered because of the game line (either its previous state or its new status quo). Goodkind, on the other hand, I can identify part of the reason. A good chunk of his books started being political in a way that wasn't seamless or obvious. That's not the only reason though, and his second and third books were similarly okay, just not packing the wallop his first novel had.
I think the problem is prevalent in genre writers (who write much of the same thing over and over again), especially successful ones who are supposed to come up with dozens of repeat successes, but not limited to them. One of the authors I read early on as a kid was David Eddings and by now, he's published a lot of books. Unfortunately, I also think he's been recycling ideas and characters and by the time you release your 24th book, it's obvious that you can only do so much rehashing.
Of course that's not always the case. Terry Pratchett, despite being quite repetitive with his Discworld novels, somehow manages to save something new and improve. In fact, I admire Pratchett because there's a clear evolution of his writing. The weakest Discworld novels in the series in my opinion were his earlier ones and some of the best are the ones that were more recently released.
Neil Gaiman, while not the most prolific author when it comes to releasing novels (not that he should be), manages to write something very different from his previous novel, which I think keeps his work refreshing. There's constant -reinvention and he's not trapped in the genre glut (in the sense that they're still writing in the same world and telling the same type of stories) that some writers are in (or choose to be in).