You may be running into married segment logic. Fare bucket availability (and price) is determined by the overall route (origin and destination), not by the individual segments that are pieced together to form your itinerary. (There are exceptions to this statement but they don't come into play here.) So although there may be lower fare buckets available MCO-ATL and ATL-IAH, the lower fare bucket may not be available MCO-IAH.

Then again, DL.com has earned its poor reputation. I suggest searching on a third party site like Kayak, Orbitz or Expedia to see if you can find the lower fare you booked before. If you can't, you are probably out of luck and will have to pay the higher fare if you want that last ticket.

Also, for the future, I strongly suggest booking everyone's tickets at the same time. There is no guarantee of availability of fares or awards or even seats from day to day or even from hour to hour.