I had an interesting experience booking an OZ flight this week.

ANA was showing an available seat in C and Y
I called BMI, they couldnt see the seat, so I checked on Aeroplan, no seat!

Ran Award Nexus, and lo and behold, the seat was visible to ANA but not to the Aeroplan search.
Ran the search again, and presto, both C and Y now visible by Aeroplan.
Ran it again, seats gone
Ran it again, seats back!

Called BMI to book them, they didnt see them again, I told them to run the search again and there they were!