How do i trigger the gracedia flower in Pokemon Heart Gold at the Flower Shop?

i have a legit shaymin that i got from Oak's Letter and caught it at flower paradise, WHICH TOOK FOREVER hahah, but anyway i traded it to heartgold to train, and when i brought it to the flower shop in goldenrod, nothing happened. now i saw that it can be from the gamestop event too, but i also saw people who got the flower when they caught it at flower paradise in PDPT as well, helpp!