Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
What specific game is it you are trying to use your gift card with?
Also, do you play the game on a computer; PC or Mac. Or a phone; Android or Iphone?
And finally, when you play the game do you play it through any specific sites, such as facebook, or wildtangent or pogo?