You can add an additional RSVP question for your guests to answer on your Enter Details page.
Under "Event Page Settings," click the blue "additional options" link, then choose "Add additional RSVP question for your guests to answer." If you have multiple questions, you can add them all to this field (usually numbering them is the best way to keep them organized).
When your guest replies with “Will Attend”, this question will be shown automatically. If your guest skips answering the question, they'll be prompted to answer it each time they view the card until they do.
This is what the question looks when viewing the card: