I have had this happen a lot. Sometimes, it doesn't mean the fare is gone. Try searching under schedule/price or just schedule and you may be able to pull it up again. Also, instead of using the selection "anytime", stick in a time that matched one of the flights you knew was included in the discounted fare.
One of the reasons, I was told by the Elite line, that this happens is that there may be only one or two seats remaining at that price and someone is purchasing at the same time you're searching. However, the reverse can happen, a seat can open up again.