User blog comment:IamJakuhoRaikoben/Sorcerer Magazine, Issue 45: September 2015/@comment-94.71.39.253-20151004062152/@comment-25324252-20151004063649

Wendy can freely activate DF at will. In order to summon the CSK, Lucy has to break a key. She only did so the first time out of complete desperation. She had a choice, die and let all her friends die or sacrifice one of her spirits to save all her friends. She went through an emotional hell afterwards. I doubt in a friendly bout with Wendy she would be willing to break another of her keys to summon the CSK.