I had the same issue when flying SIN > LHR earlier this year. My wife and I (both silver) were flying together, but on separate reservations. Both were offered the same POUG on the return and tried to book it. It turns out I managed to upgrade and she couldn't as it failed at the payment page, repeatedly. Several calls to BA followed, including a plea for help to Nicci, but in the end we had to travel separately. The most bizarre thing was that, while on the phone with the BA agent, telling me the offer was never made available to my wife, I could see the POUG on her MMB!

In the end I had to give up my seat in CW and slummed in WT+ so she could enjoy a more comfortable ride...