After you finish the charms challenge, go outside and listen to the cut scene about Neville. Now go to the chest and open it to recieve a pack of cards. Save your game and quit. Now continue your game. You should be just coming out of the charms challenge again. Check your cards. You will see that you still have the ones you got from the chest. Go out of the charms room and listen to the cut scene again. Turn to the chest. You'll see that its closed again! Open it for another pack of cards! Do this until you have every card.