The episode of "American Pickers" that Danielle Colby didn't appear in was "Bigfoot in Alabama" from Season 1. This episode focused on Mike Wolfe and Frank Fritz when they go to Mississippi to visit John, a motorbike collector.
After purchasing a motorcycle, they head to a car collector in Alabama where they find a frame of a rare Italian scooter, which they purchase for $40. On their way out of town, they stop at a scooter shop and sell the frame for $250, making a nice profit.
Perhaps Colby does not appear in the episode because the show was in its first season and still experimenting with its layout. The reason for her absence is never explained, and Colby has appeared in every single episode since.