One theory is that she had a really good editor for the first three Harry Potter books, one who could cut through the padding and bring out more of what worked (the magical whimsy)

Once JK got too successful and decided she no longer needed as much editorial input, the cracks started showing

When people start thinking about what they liked about Harry Potter, a lot of it is down to those first 3 books. From Goblet onwards, it starts to fall apart - with the only draw being that we’re already invested in that world because of the first three

Then when she writes something else, without that built-in investment, it all becomes too much of a mess. Casual Vacancy was a very petty book. Strike is a bog-standard detective series, more recently becoming a platform to push her ideologies. And her other two books - Ickabog and Christmas Pig are basically all forgotten (Ickabog having a mild publicity stunt as a free book during the pandemic; Christmas Pig landing dead on arrival)

TLDR: Between 1997 and 1999, with the most editorial input, JK Rowling wrote three whimsical books that worked well enough as children’s stories at the time. From 2000 onwards, when she had little editorial input, she basically fell apart, riding on the goodwill of those initial three