They were right as far as location goes; it's from a woman in the pokemon center in Lacunosa Town. There are 3 things I can think of;
1) Make sure Shaymin is first in your party.
2) It must be an event shaymin with a "fateful encounter" tag
3) It cannot be nightime or the Shaymin cannot be frozen to become sky form. (I'm not sure if this affects recieving the flower, but it's worth a try)