Writer's Fallout Shelter

An agent has responded to my story. This is the first one that hasn't sent me a standard form letter of rejection. Gist of it seems to be that he likes my writing and my characters and settings do pull a reader in, BUT it lacks "spark". The central plot seems to be still unknown to him after (think he said 14 chapters). His assessment is that without "spark", it won't sell. However, he inserts a disclaimer that he could be wrong.(but from his tone, it does sound like a disclaimer and he is pretty confident.)

So, ... I guess the question is now "where to"? Do I slink back to the drawing board? I'm convinced I need to attack something, yet I know not what.

-Renegade