One of my friends said that an author's first book will always be their best: it's a culmination of life experience and artistic drive, whereas any following work won't have the same fire.
I didn't agree at the time, yet I often read books that I love, only to pick up another by the same author and not find what I'm looking for. It's like ordering a good meal from a restaurant, only to return and not like the next. But maybe I'd like it if I tried that one first; maybe it's just a matter of comparison.
And there's also the argument of growth. Any specialization, especially artistic ones, take practice to make perfect.
But, in his favor: I like seasons 1-3 of Better Call Saul more than 4-6. I prefer Dostoyevsky's earlier work. I was disappointed by Blake Crouch's latest novel, and couldn't get into Stanislav Lev's final pieces. I have mixed feelings about Panic! at the Disco's new album, and I think St. John's first novel was her best.
It's an argument of refinement vs passion, desperation vs determination. Artists tend to create not because they want to, but because they need to; and once that first novel or painting or movie is out there, can anything compare?
I'll always return to authors I love, but I have to learn not to expect the same experience over again. It doesn't mean they haven't improved their craft, or that they can't ever create anything as good: it's just hard to see a second creation out of context from the first. The first bite of a meal is always the best; doesn't mean you stop eating.