When an author is having trouble writing three good paragraphs for a query or writing a two page synopsis, no matter how often they rewrite, does it mean the problem might really be in the novel, not in the summary? Shouldn't all well-structured novels have a central theme or plot, or are some great, fun novels just not "summarizable"?
I believe anything can be summarized in the usual query length. The key is not the length of the summary, but finding the right information. The following three plot descriptions progress from vague to general to specific. The first is the longest, the last is the shortest. They all describe the same work.
Some interesting stuff happens to a bunch of characters, and one of them has to make a tough decision that will affect a lot of people.
When a man's ex-lover comes back into his life, he must decide if he wants her badly enough to risk more than just her marriage.
Rick, a cynical cafe owner in Nazi-controlled Morocco, must choose between his feelings for his ex-lover Ilsa and his once-strong sense of patriotism.
Given another sentence I would identify Ilsa's husband as a heroic figure the Nazi's would like to capture. Given another, I would bring up the letters of passage. And so on. Possibly a good way to write a summary is to summarize the book in one sentence. Then add another, as if two sentences were all you got. No cheating by saying I'll add x in sentence 2 and y in sentence 3. Choose between x and y. What is the most important idea to add to sentence 1? Keep going until you reach your predetermined maximum, which is probably nine or ten in a query letter, twenty in a short synopsis, etc.
You can't be too specific without writing too much, and you can't be too general without becoming boring. If your book is extremely complex, you may need to be more general than you would with a simple, straightforward plot. You have to find the right mix for your book.