I hate to say, but it really could be her personality. The Chi I first brought home (last year) Kip, was terribly shy. Partly personality, partly lack of socialization... When I saw him last, he was coming out of his shell as long as he was with other dogs. Jerry is fairly outgoing, as long as he is standing near me or Tabitha
Tabitha, on the other hand, will seek out new people, new territory, new anything. With love and a whole lot of patience, you can probably bring Pixie out more but, just like people, she will generally stay true to her nature.