The last time this happened to me was earlier this year when I was looking for a trip to HND/NRT. Delta.com kept telling me "We're sorry this fare has just sold out" for a period of several days. I tried calling the medallion line and web support and neither was able to honor the lower fare. In the end I discovered that expedia had that fare available and I was able to book it through them with no issues. You may want to to see if you can get the ticket for that price on Expedia, Orbitz, Travelocity, etc.