Have you ever come across a story which has a strong beginning, exciting tension, and believable conflict, but, once you get to the middle of this book, you have to force yourself to keep going? You suddenly lose interest, and you can't figure out what has happened to this story that started out with a bang.
Some authors have a hard time keeping the momentum going all the way through the book. They may have a terrific ending planned, but they have "sagging middle syndrome."
How can you tell if there is a sagging middle? It does not necessarily have to come right in the middle of the book. What is important is that the reader has lost interest in the story.
When writing, don't just be satisfied with a strong beginning and fantastic ending. Trying to "fill in the middle"can leave the book sagging in parts. In order to prevent this from happening, one of your main characters needs to have something unexpected happen. Conflict is generally the cure for what is ailing in a book. Be sure the conflict is tied in to the goals and motivation of your characters, and you will create a story that will have the reader hanging on for the entire ride.