Ouch, did you pay using another gift card or credit card? If it is credit card I would just dispute the transaction and let them handle it from there, would be more tricky if you paid for it with another gift card.
Win some and lose some I guess.
I did notice nowadays most of the stores have a sign saying please use the manual counter for purchasing gift cards as sometimes they are not activated properly, albeit it does have the drawback that most staff won't let you buy gift card with another gift card.