The thing is, neither you nor I shall be sorry. I promise. You want an agent who adores your writing, who is passionate about your book and who feels confident that they know how to sell it. No matter how good your book is, an agent who feels lukewarm about it will not be an asset to you. We say this often, but it is true- agents turn down saleable projects because we don't have the right experience or enthusiasm for the material. All agents might turn down a project that goes on to be published and performs well, and think maybe we should have rethought taking on that project, but this business is so subjective, we have to go with our gut. We did not love it, so we did not take it on, and someone who loved it sold it and another person who loved it bought it, and that is probably why it is doing so well.
There is a preoccupation with the idea that agents are often proven wrong and are sitting at our desks ruing the day we tossed that manuscript aside, and I can see why it is an appealing vision to someone in the midst of the frustrating, lengthy process of looking for the right agent. At every conference I have attended I am asked if I passed on something that I regret. I do see why you want to know, because we agents deal with rejections, too. We sometimes lose a new client that we want to another agent, and we have editors rejecting our submissions. We understand. But we have to focus on clients that want us as much as we want them, and on finding the editor who loves the project as much as we do, not on the editors who don't.
So please, don't call me to plead your case, or keep sending me the query I passed on. I am not going to change my mind on that particular query (however, if the agent said she would be willing to read again if you revise down the line, you should). And you can always query with a different project.
Instead of resending the same query over and over to the same agent, spend your time looking for the right agent, not trying to convince the wrong agent that she is right. Spend time on perfecting your query letter and on perfecting your manuscript, to give your project the best chance of success with the next agent who reads it.
Here is the hopeful side of this-- many successful books were once rejected by lots of agents, which means that even if one agent doesn't love it, another will. And that one or more rejections is not the end of the road.
But my no, on that particular query, is still a no.