This has to be one of the most difficult things an author needs to do. It's something I find very frustrating as well, and that's not the agents fault, it is something to do with the way I read, understand or interpret the various 'guidelines' they so helpfully publish.
For a start I have never been very good at 'selling' myself. I'm not very good at 'selling' anything actually, so there's the first problem right there. Pitching to an agent is a 'selling' exercise. You have to come up with a sales pitch that makes them want to read the rest of your story - and then sell your story and the characters to a publisher. As one put it too me rather kindly, he has to be able to 'like' a character to sell them. If he doesn't immediately think, 'this is someone I'd like to meet in person,' it's a waste of the his time and the authors taking it on.
That presents something of a challenge for a writer. Quite often - certainly in many of the books I've read - you don't really get to know the main character until about halfway through the story - and even then, you may really only 'know' his/her reactions, sexual preferences or role. So the challenge for a new author is to give the reader (and of course, the agent and publisher) a strong introduction to the main character in the first few pages. This can be a challege because you're also trying to set the scene and pull the reader into the story and the world it is set in. One successful author I've met told me it took him years to find out how to do it all effectively - and now he doesn't have to.
To an extent we, as writers, are also up against the 'Twitter Mentality' which a fellow teacher and technical author sums up as the "tl;dr" syndrome. It stands for "to long; didn't read" and is a response he got to a request for instructions on a rather complex operation the sender asked for.
Agents have time constraints as well, after all, they have to get out and sell the work of their clients, so the pitch has to be eye-catching, succinct and grab their interest. According to one agency that must all fit on one side of A4 paper as a covering letter accompanied by the first three chapters of the book. Right. It's a challenge, it is possible, and it shall be achieved.
Time to get back to refining my pitch. Time to go for a "gold." As my former boss once told me after I'd been beaten in competition for a promotion - there are no prizes for coming second.