Remember way back in grade school when we first did book reports? It was probably our first foray into essay-style writing, all based on what we had just read. Our first ones were probably not masterworks by and sense. "This book is about a lady. She has a horse. She loves the horse. They ride around the country..." Yadda, yadda yadda. We basically take a work, process it through the mind of a ten-year-old, and retell it in about one-hundred words. Nice work. Trust me - it's important that we take this step.
As adults, we probably read more book reports than we write, but our reflections on them become explorations of what ideas the book explores. How it discusses them is not as important as what it discusses. Just like a good movie review, it should tell a lot about themes and concepts but be very thin on actual blow-by-blow details. The actual story is for the reader to consume. The review tells that reader whether it will be anything more than just a superficial romp in the park.
Do you see where this is going yet?
When we sit down to write some masterwork, we should already be able to write our book reports on it. Not just the simple ten-year-old version, but we should have our junior-high version and our adult book review versions as well. This may sound odd, but it's true - each of these has a place in our writing, and if we can't write them before we write the book, we probably aren't ready to write the book.
In business, these three concepts are usually discussed as: The Elevator Pitch, The Proposal Pitch, and The Presentation. They are the same as the ten-year-old, the junior-high-school and the adult reviews - the first one says what happens, the second one expands to ideas, then finally themes. But in business or in writing (and always in the business of writing), they are all necessary for the person to know before anything is done.
If you are working on a major project and find yourself hung up on some part, put these ideas to work. First, write your quick run of the story - one-hundred words to tell the adventure. If you can't package that briefly and succinctly, you are having problems. Second, go to junior high and write about what the idea of the story should be; what will engage the reader besides basic actions, character and dialogue. No more than 250 words should do it. Then write your book review, exploring in about one-thousand words the major interactions, concepts and conflicts, and what should drive someone to pick that book over all others.
If you can't write these, well, that's probably the problem - you don't fully understand the story. If you can write these - and actually do write all three of these - then I guarantee you will resolve what is holding you back.
Of course, if it also inspires you to read Lord of the Flies again, well, that might help too.