Sounds like a married segment issue. Are you trying to upgrade one flight or one leg? If I understand married segments correctly you need to upgrade the entire segment not just one leg, so all flights need availability. E.g. If you are booked FRA-FCO-CPT, and you only want to upgrade FCO-CPT (or there is only availability on FCO-CPT) then you'd come up against a married segment error.

More award availability restricted by married segments / connections

It certainly applies to AA, didn't realise it applied to A3 too.