1. Your Query is Too Long
On the last day of the conference, there was a query letter gong show. It was hilarious, but I learned so much about what agents and editors look for in a query. The most consistent problem? Queries that went on too long. Many times the agents were intrigued by the initial premise, but grew frustrated when the query droned on. Keep the description of your novel to one or two concise paragraphs--leave something to the imagination.
2. No Gimmicks
In general, the agents preferred to have relevant information like genre and word count in the query's opening sentences. Cold opens only work if the genre is immediately obvious. Likewise they didn't like gimmicks like too much slang or confusing jargon. Clear explanations that give them a sense of the writer's characters and voice got the best responses.
3. Agents and Editors Get Rejected Too
It's easy for writers to get frustrated. Since we get so much rejection, we often forget that agents and editors get plenty of rejection as well. An agent can find a book they love, do their best to promote it, and get rejected by editors and publishers again and again. That's one reason they advise writers to keep writing more books--if the first one doesn't sell, maybe another one will. Editors may love a book, but find it gets rejected by more senior editors, or the marketing team, or some other higher up. This is one reason agents only rep books they love: they have to be willing to risk rejection and go to bat for it.