@AnnaLandin and @MrKyleRose summarised it nicely methinks.
For one of my university courses we have to come up with several pitches for several different story ideas per term. I've found it's far easier to make up something from scratch than to take a beloved, well developed idea I've been working on for years and strip it to its core.
Part of the reason doing synopsis and elevator pitches is so hard is just that. You have to really know the central thing that makes your story stand out from the others and put it out there. It is said that the industry puts so much emphasis on elevator pitches because if you can only tell your story with a rambling essay then you don't actually know it! The elevator pitch is the core idea, and once you figure it out it's a good idea to stick it somewhere and remind yourself of it all the time to make sure your story doesn't lose it's plot or theme.
That epic star-crossed romance you can't wait to bring to life? That ancient feudal vendetta that has torn the world in two since the beginning of time, with all those juicy historical details that you have rehearsed the telling of ten thousand times over? They're all secondary details that will be what makes the story memorable once you've proven that it's worth reading... and not just another Romeo and Juliet retelling with a new paint job.
For this kind of thing I know there isn't really universal advice, but one way of looking at a synopsis or summary that I found really helpful was to go back and recall the initial idea that you had - that very first little "what if...?", that you might have had that conceived your story as it is today. Likely that idea you had initially will also encompass some information about your main genres and the relationships that would be born out of it. "What if an 18th century highwayman fell in love with a nun?" Bam. It's got the characters, it's got the setting, there's romance, there's conflict. To make that into a synopsis we just change it from a question and add some nicer language or something, "In 18th century England, a down-on-their-luck trio of highwaymen lose all hope when their leader falls in love with a nun." Or whatever. It's a starting point. Sure, lots of the finer details are missing, and many a first 'what if' isn't quite that helpful, but it's some bare bones to work from.
And if you can't recall if there was such a thought, if your story just progressively developed overtime from some character you created, another thing I found really helpful was to look at your story and think "what one, central element makes this different, unique, cooler or more memorable than all the other stuff out there?"
Which can be a pretty hard thing to figure out, I find :< especially when it's the bonds I've formed with the world and the characters that makes me love the idea rather than any unique merit of the story line itself. After all, nothing can ever truly be original, everything that can be done has been done, we can only twist it a little bit to tell it in a new way.
People like familiarity to a certain extent, but familiarity with a twist. @MrKyleRose has a great example here "A sentient zombie holding down an office job in New York City, because even the dead have to make a living."
Zombies, we all know that trope. They've been everywhere lately! It's just another zombie story, isn't it? But wait! A zombie doing boring mundane everyday things because he needs to 'make a living'? Hah! That is beautiful, unexpected, memorable, quirky.
Introduce a familiar concept, give it a twist, and make sure you introduce a character, a conflict and a setting in the process.
Personally I suck at these types of things, coming up with a bunch of them is the hardest part of that university course for me. I die a little bit on the inside every time and spend several days carrying a notebook with me everywhere while I look at every single thing I see and desperately try to squeeze a story idea out of it. My webcomics on here don't even have very nice ones at all xD I keep rewriting them to death and being annoyed and just bah! Especially with shorter ideas D: there is only one interesting conflict! And it comes in pretty late! How on earth can I make a compelling elevator pitch out of that!?!?
Anyways that's my two cents 0u0 I'm so sorry I typed so much D:
tl;dr: Try find the central unique aspect of your story idea or the initial concept and conflict that inspired it and put that into a sentence. Elevator pitches are hard man ;A;