"Not properly activated" iTunes cards

I was recently given two iTunes card, each worth $10 for Christmas, and neither of them seem to work. I've attempted to enter both of them on the App Store multiple times, but this message appears when I do:

The Gift Certificate or Prepaid Card code you entered has not been properly activated. Please contact iTunes Store customer support for assistance at http://www.apple.com/support/itunes.

My mom, who purchased them for me at Walmart, told me that the cashier had checked them out wrong or something, but that she would take them back to the store to ask what had happened. I was wondering if anyone could help me with this problem so she wouldn't have to make an extra trip back. I've noticed this problem has happened to some others, but they haven't gotten any answers, so hopefully someone out there knows how to help.