Give that agency a second shot. Just be up front about it: “You passed on a previous submission of mine, but I do believe we’d make a good match and hope you’ll take a look at this new project, which is great in all these ways.” Put this in the second paragraph of your query rather than lead with it. If the agent’s rejection letter included specific comments about improving your manuscript, thank him for taking the time to give the feedback and tell him that you’ve incorporated his recommendations into this new ms. If you get a second rejection, then it’s time to move on because this agent isn’t responding strongly to your work. You want someone who’s as passionate about your writing as you are.