Strange Award Booking Issue

This one has me baffled.

I'm holding space SFO-HKG-BKK-VTE. Everything in F except the last segment due to lack of availability at the time I originally booked months ago.

Yesterday the website showed space in F so I tried to change the ticket. No go. I called Res and they couldn't get it to go either even though the computer showed space. Finally the agent told me to book it as a separate award ticket and she would fix it (ie make the change to the current reservationI. I did. She then called TG and they said they couldn't find it.

I was ready to give up but today I called TG and they show the reservations I made and they are perfect. So I called UA and said what had happened and they said they won't fix it. Huh? I did exactly what their own agent told me to do. The only reason she didn't fix it was that the TG agent couldn't see the res probably because it hadn't come over yet.

Here is what we know. There was award space. I should have been able to change my existing reservation to F because it was clearly there (I even took screen shots) but the system wouldn't let me. We are positive the space was actually there because it DID let me book a new, separate award reservation just for that leg BKK to VTE. Since the new reservation uses the same booking class as required to change the existing booking then clearly the space was there not only because I saw it but because it actually took the booking.

So right now I have my original booking for the entire journey and my second booking for just the BKK to VTE leg. The first one has us in coach, the second in F.

What to do? No one as UA seems to have a clue even though I did exactly what they told me to.

Any suggestions?