If you look through the other posts you'll get lots of explainations as to why you may not be seeing the (AMAZING) Cheapflights offer.
It is generally only available at certain times during the day. In the Midwest (or in MN - where I'm from) I notice it becoming available around 330am-4am every morning. Once it becomes available, I THINK cheapflights only allows a certain amount of people to use the offer, and once the quota is full it becomes unavailable until the next day again. So you probably aren't seeing it because you happen to look for it when it is not available. I'm not sure how long it typically stays open- maybe someone else on here knows?
Good luck! Hopefully this helps you get your Cheapflight credits! =)