A number of members have reported trying to add a FF seat to an existing reservation when they can see a seat available on Qantas but the call centre agents can't. I ran into this today, not on a oneworld classic award but a regular frequent flyer booking on oneworld carriers and I think I may have some insight.
I was trying to change an existing, 3-leg itinerary ex-Paris. The already booked reservation included a LHR-HEL and that's what I wanted to change to a few hours earlier. I could see the seat on Qantas and ExpertFlyer showed one seat left. The agent couldn't see any seats in either class. While we were on the phone, I booked online the seat she couldn't see and expressed my aggravation at having to do it this way as I was now unprotected on an onward long-haul leg.
The agent looked into it and her conclusion was when I asked her to make the change, she was looking at availability priced in euros, not British pounds. I asked if this was a point of sale thing and she said no, just the system or screen they use. When she later looked for seats priced in pounds, she also couldn't see any but that's likely because I took the final seat.
I think the takeaway here is that if you're trying to add a flight that's priced in a different currency to what the existing itinerary is priced in, you may get an error over the phone. I don't know if asking the agent to search for availability on the 'screen' that shows availability priced in the original currency would help (or even be possible), but it might be worth a shot.