I think he, as so many, changed from storyteller to Author; he became, to my mind, far more concerned with the craft than the art. Asimov, for instance, or Bradbury, always retained the wonder of the storyteller, I think. None of that is to say Zelazny was not a giant, and a shaper ... just that I think he peaked long before his untimely demise.
I know what you mean, but I'm not sure it's entirely true. Take "Creatures of Light and Darkness" for example. It was first published in 1969 which places it in a particular period of experimentation in American literature that produced works by Thomas Pynchon, Joseph Heller and Kurt Vonnegut.
I don't mean to be condescending, but many readers of more traditional sci-fi have had difficulty with "Creatures of Light and Darkness," and rate it low on the list Zelazny works.
It is a difficult book to follow due to the experimental style Zelazny employs but it has, I think, all of the other merits of his more accessible books.
It seems to me that Zelazny was always quite interested in the craft of writing, I just think that he lost, in later years, whatever the spark was that made his earlier works so special. Maybe his imagination ran low, or his personal life became too content.
With Amber, its pretty clear (at least to me) that he had embarked upon a journey with very little knowledge of the destination or the places he might visit along the way. Many authors claim this is the case for them when they begin a book and that it works out just fine, but I'm not sure such is the case when the work is a long series of books (particularly if the series becomes a relied upon source of consistent revenue). In any case, only a Zelazny fanatic would rate the latter Amber novels as highly as the first few, and the first turned out to be the best of them all.
I don't know that I ever read a Zelazny book (and I believe I've read them all) that I didn't enjoy, but there have been a lot that I found disappointing if only because they didn't match the earlier works.
"Isle of The Dead" was another one I enjoyed quite a bit, as was "Jack of Shadows" mentioned by Brandon, and also a bit more experimental than most of the others - with the exception of "Creatures of Light And Darkness."