No clue how to adapt a book into a film

Rating: 5/10 (22 July 2016)

Before I start I need to bring up that I am not a fan of the book or the film. However there were aspects of the book that were good and the movie made a mess of them. For example, the book's characters were great. Gally was a great antagonist, but he became a whining bully who couldn't get his way with no notable qualities.
Some fantastic scenes in the book were also altered in the wrong way or skipped completely. Alby's death in the novel was powerful and well written. In the film he is picked off like everyone else in the Glade, except he gets about ten seconds of "dying time".
My final complaint is how the changes in the story line and time line opened up plot holes that didn't exist in the book. In the final act of the film and the final chapter of the novel Gally appears at the headquarters of The Glade. In the book he had been dragged off by a griever several days prior, so it made some sense. In the film Gally was still in The Glade when the other characters made their escape attempt. So how exactly did he get to the end without anyone noticing?.
The only reason why I didn't rate this lower is because of the well done action.