In Chapter Six of The Great Gatsby, Nick narrates the story of James Gatz, Gatsby's past. This is an opportune time to inform the reader of the history of Gatsby/Gatz because it is in this chapter that the present links back to the past: Gatsby's reunion with Daisy. "Then he kissed her. At his lips’ touch she blossomed for him like a flower and the incarnation was complete." The "incarnation" refers to the transition (from Gatsby's perspective) of Daisy as this ideal, romanticized vision of perfection to a wonderful yet realistic human being. Gatsby is thrilled with this reunion but the reality of it does bring him back to earth. For Gatsby, Daisy was the always elusive American Dream. He is happy but since Daisy can't possibly live up to his ideal vision of her, that ideal vision still eludes him. And, over the course of subsequent events, Daisy (the dream) turns out to be too elusive even for the determined Gatsby.
When Nick tries to remember a phrase at the culminating moment of this novel, he might be inferring that a verbal description of Gatsby's initial triumph (via the kiss) is, like the ideal American dream, equally elusive. The reality of achieving this dream is elusive in experience and words.