If you have written evidence that the fare was refundable, I would request a refund now. Inasmuch as you agreed to accept a credit there will be resistance. Bear in mind that the refund will ultimately come from the airline, not from the agency, and they need to do the work to obtain it for you. Is there more than one airline involved? If you dispute the card transaction do it in writing as required by law. Telephone calls seem easier, but you need to force them to explain their decision in writing.