Apologies if this is covered somewhere else. I searched a few different ways but didn't find it.

I just completed a trip to London and have two more scheduled over the next few weeks, all from NYC. In each case, I tried to book specifically the legacy United planes with Global First, but in all cases, United would not sell me a FC seat. Even with some flexibility of times, I could not find a FC seat for sale EWR-LHR on my dates.

The weird thing is every flight I checked, the seat chart showed FC half or more empty. I chased this up with the GS desk, via email, and they seemed puzzled as well. They confirmed what I was seeing namely largely empty FC cabins but no tickets for sale. They had no idea why.

For this last trip I made the mistake of just buying a business seat and waitlisting first, assuming it'd clear. It never did, and I was stuck flying backwards in the old United business seat, in a red eye. So for my next two trips, I've booked British Air FC instead, but all else equal, I'd much prefer to give my PQDs to United.

Any ideas?