In many cases when a reader puts a story aside because it 'got boring,' the boredom arose because the writer grew enchanted with his powers of description and lost sight of his priority, which is to keep the ball rolling.
Stephen KingRemember in elementary school you were told that in case of fire you have to line up quietly in a single file from smallest to tallest? What is the logic in that? What, do tall people burn slower?
Stephen KingThat's the curse of the reading class. We can be seduced by a good story even at the most inopportune moments.
Stephen KingLet's get one thing clear right now, shall we? There is no Idea Dump, no Story Central, no Island of the Buried Bestsellers; good story ideas seem to come quite literally from nowhere, sailing at you right out of the empty sky: two previously unrelated ideas come together and make something new under the sun. Your job isn't to find these ideas but to recognize them when they show up.
Stephen King