I know a few people who worked on this movie and it wasn't Rowling's fault. Her original script looked VERY different from the final movie. Her script was very dark and she loves characters and worldbuilding too much to keep it short enough to fit into a movie.

Apparently the crew was fixing and rewriting the story less than two months before the movie was released in order to make it less dark and more linear. Even with their edits it still didn't fit.

Rowling should've written the stories as full novels and WB should've been patient enough to let her do that. Instead, WB threw money at her to rush these stories out and then tried to fit them into a movie format (which doesn't work for Rowling's work.... Only a few of the original HP movies did justice to her work).