The card packs give you a chance to get one of many items and there is no guarantee that the one you wanted will drop for you. However, your chances of getting the item may increase if you try multiple times. You can check the drop rate of each pack in the Shop. Or you can tap on the ''i'' icon on the pack in the rewards list in the Limited-Time Events menu to see the possible items you could get from the packs. As long as the card packs always give one of the items that are listed, there is no error occurring in the game.