After the popularity of The Secret, many people are familiar with and have tried manifesting. Basically, the premise it that if you want something, envision it, believe you will have it, and it will come. In practice, sometimes it works and sometimes it doesn't.
Why doesn't manifesting work all of the time The Invisible Path to Success gets to the root of this problem. The author presents a variety of different possibilities. For instance, often you may say one thing aloud but add a silent PS at the end. You might ask for a raise but immediately after asking think about the unrealistic nature of this request. This tells the universe to cancel the order.
Another likely scenario is that you ask for a billion dollars, a brand new Ferrari, a supermodel wife, and a mansion. Although these things might be great, for a while, the odds that you were put on this earth to do more than lounge beside the pool and drink margaritas. Therefore, if your request doesn't reinforce or otherwise aid your life purpose, it will again be immediately cancelled.
I absolutely love the movie analogy that the author uses to explain these concepts.