There is a pact between a reader and a writer, and when it is established with credibility the reader will read and commit to the writer's offering. Likewise,the writer will, on their end, agree to give the reader something for their time--a story, an idea, a way to look at the world.
But sometimes we don't quite hit our mark, and we end up failing the agreement with our reader and well, this is a bad thing. So in order to prevent this from happening, we need to look at what our readers expect and how we might be failing their expectations.
1. We pledge an epic eruption at the end and all the reader gets is a tiny thunderclap. This is also known as not bringing the heat. We cannot build a climax for three hundred pages and not give them what they paid for. So, if you hint at a big finale, don't close the curtain before the job is done.
2. We commit to paper people who are true to themselves as we have written them. This is known as making characters stay the same throughout the book's journey. But sometimes, they just don't behave. They grow and change and act like someone else by the end of the book. That's really okay, sometimes they do change just like we do. But when they are zombies who don't know who they are and why they are doing what they do, well, that is not okay.
3. We vow to keep our chapters succinct and to the point. And then, the reader decides that they know exactly how it is going to end, and quits reading. Well- we should try to keep things interesting and not too easy to figure out. Part of the element of surprise is well, the element. Don't make out like the story is not going to move along in it's natural rhythm. It will. And it's okay to allow it to seep over the edges of convention as long as it doesn't jump in the river and swim all the way downstream.
So finally, try to make an earnest effort to give the reader what they think that they are getting when they read your blurb. But don't forget to throw in that special sauce that makes them say they were pleasantly surprised too.