My agent didn't sell my manuscript either, but that doesn't mean the agent is a failure. You need to sit down and take an honest look at the situation. What publishers did she submit to? Did you get any feedback from the editors? And here's the dreaded one that's the hardest to think about. Was your novel just not quite good enough?
My novel went to some of the top editors in the genre. (How do I know? She told me who she submitted to and then I researched the hell out of them.) All came up with relatively consistent responses. Much as I don't like to admit it, the editors are right. My manuscript is just not quite up to publishing.
But my agent believes in me. And it sounds like your agent believes in you. I also believe in my agent. It sounds like you don't believe in your agent. Rather than hunting for a new agent, you might want to write your next book and make it so strong there's no way an editor can turn it down. That's what I'm doing.
Best of luck,
Jim Clark-Dawe