i redeemed a gift card but itunes won't let me use it.

I bought a $15 gift card from best buy and added to my account to redeem. I went to buy a movie and it just keeps telling me that my account is insufficient balance. the movie was $14.99 and itunes was showing my account contained $15.67 so why doesn't it let me purchase the movie? I entered my credit card number and now itunes has no record of my gift card and when I tried to redeem the same gift card again. it says it's already been redeemed but It has no record on my account about it. ??