I had this effect a couple of times. A very nice (no irony here this time) employee of Delta told me, that this usually happens when a very large number of ST Medallions (esp. FO) is booking the lowest classes and the system is unable to clear any updates right away.

Also, there may be deeply discounted fares still availabe, the only issue is that they dont upgrade FO's in advance. Most FO upgrades are awarded at the gate, so good luck there!