Nobody, not even the best of entrepreneurs, were born with the art of pitching in front of investors. Its a process that can be learned only by interacting with potential investors, getting them to understand what do you wish to do, and how do you wish to do it. But, most entrepreneurs, who otherwise might be running startups successfully, fail to do this. And, that is mainly the reason as to why they fail multiple attempts of getting funded.
Below is a slideshow by Alexander Jarvis, Operating Partner at Jungle Venturesthat would help you learn the basic structure of how a pitch should be designed to make an impression in front of the investors.