This happened to me last week from JFK-TPA on the 26th. I'm GM, but did not clear at 72 hours, even with 11 seats open in FC. SMS told me that Delta was holding the seats open for revenue sale because Y was heavily oversold. I argued for a bit, but obviously it did no good. I ended up clearing at the gate, being #1 on the upgrade list. It doesn't seem particularly fair, and it is certainly frustrating, but not much that can be done.