You may have selected a video or card that does not feature animated mouths or mouth movement.
If you watch the preview of the card before clicking the "Make Card" button, you'll be able to see if that card features animated mouths or not.
It may also be an issue with the face you're using. You can try making the same card with different faces test. If this is the case, making a new version of the face should resolve the issue.
If neither of these solutions work, please Contact Us with a link to the card and a description of the issue so we can investigate.