why snow fairy cant vượt qua, cross the border, while in the 1st movie they came to pixie dust tree?

i watched Tinkerbell: Secret of The Wing last month.
And today i watch tinkerbell 1st movie.
i am little confused. in the Secret of The Wing, snow fairy cant vượt qua, cross the Border. But why in 1st movie Cera (snow fairy, i know her name in tinkerbell Mất tích Treasure NDS) and minister of winter came to the pixie dust cây when tinkerbell looked for her talent?