And the logic is....?

Flying out to LA on 1/10/11 on an S PE and back UCS on 7/10/11. I've asked FC if I can upgrade the outbound leg to UCS, and was told nothing available but check the web site for availability day-to-day.
I've checked 2 days running, and there is availability of miles upper class tickets both ways, but they continue to tell me on the phone that there are no miles seats available. What's going on??? ?|
I've checked 2 days running, and there is availability of miles upper class tickets both ways, but they continue to tell me on the phone that there are no miles seats available. What's going on??? ?|