My impression is that DL is holding back seats from the Medallion upgrade bucket so the inventory slides down to the GAs. At that point it enters the whole realm of GA issues that keep coming up (GAs that seem to be too harried to fully and accurately process gate upgrades and/or the various 'shenanigan' complaints).
Socrates - you may be right that DLs inventory of FC coming into the merger was lower than NW but I don't think there has been any major reduction in FC from the former NW fleet yet so the inventory to use should be relatively close to pre-merger. The one thing that has changed in a major way in the last few months was the shutdown of NWs primary IT and whatever was left of NW's rev mgmt running it. Its all DL rules now hence my thought its inventory management related.
Assuming I'm right its really too bad DL isn't thinking this through. It would indicate that they are chasing the 'high paying customer rainbow' too much and just creating ill-will and mistakes in the process.
Of course the surly side of me is rather pleased with the idea of DL having its own 'rainbow' chase. I rather suspect that the 'high paying customer' is probably about as rare as an easily found low redemption level award these days. Nevertheless when the result is a lowered customer experience its just not right.