Split The Pack
When going through flashcards rather than put a missed card to the back of the pack split the pack so that the missed card goes somewhere in the middle.
If a missed card goes to the back of the pack then chances are the students won't see that card again that session, or if they do manage to go through the whole deck chances are they won't remember what the card was. Splitting the pack vastly increases the chance that the students will encounter a missed card more than once. This means the students will pay more attention when you tell them the answer as they will probably keep encountering the missed card.