I was rebooking reservations using Ding today when one of my flights was missing from the selection list. Just not there at all.
I phoned Southwest and they told me that the inventory was being updated for that flight and that it would be shown within an hour. 5 minutes later the flight was there.
It's goofy that Southwest needs to make a flight invisible just to update the fare buckets.
That happened to me this afternoon...thought I was seeing things (or not seeing things). Afternoon nonstop, SAN/MDW, but it reappeared a few hours later.