@Sey: after they reunited at the lantern festival they hung out for a few days and then YWY left, after that they would see each other a few more times here and there...YWY didn't ask her to come with him until later at the same time Xinger was thinking she didn't deserve to be with him because he's suffered so much losses because of her and due to her tricky identity (wanted criminal of Wei) she would bring him complications when his future looks bright now with so many accomplishments...basically she didn't want to hinder his success so she thought the best thing for him would be to not be with her...for him he wanted to give her more time as Xiao Ce advised him to give her time since she went through 2 great losses in her life (breaking off a 10 years relationship with YX, mourning YWY's death for the past 2 years)...however, later she realized that she needed to live for herself for once and decided to be with YWY (I *think* this was also Xiao Ce's advice to her, fuzzy memory about this part so I would need to reread the chapters).
@Sunzbery: the book didn't really mention "why" YWY didn't contact her...just that in the 2 years he's done a lot with his life...he basically came back from the dead to become a strong force everyone had to fear...:)