Does Wal-Mart give out a slip of paper that the card got activated like Target does? It sounds as if Wal-Mart did not scan the card to activate it.
Scanning the card is not enough. It is a gift card, and like almost all gift cards it has no value until it is activated. The cashier has to slide the card through the credit card slot on the point-of-sale terminal, and then complete the card activation screen. If the cashier just scans the bar code on the card, the correct price will come up, but the card will be worthless.