I encountered this before. I flew from ORD to BOS with an award ticket and I was a gold medallian. At T-72 I was upgraded to C+ but at T24 I was not on the upgrade list after checking in. Called GM desk and they could not solve that and I had to activate at the gate. At around T2 I got to the gate, the gate agent could not handle this by herself. After about 1h of call, finally, I was on the list. She said the reason was that I confirmed the C+ upgrade, which I didn't.