Same thing just happened to me on a muti leg. DM 1MM here. I travel every week. Two the legs let me select C+ at booking. The other two legs were asking for $ to upgrade. I gave it 24 hours and it still asked. I called the DM line and they seemed clueless as to why I couldn't upgrade. They did it for me on the spot.
Yes its a Diva/first world problem but it REALLY annoyed me.