Redeeming Velocity points on Air New Zealand flights.

I've got a particular flight in mind (next June) but after calling they keep saying that there are no reward seats on the flight and flights surrounding the date. Normally I'd just let it go but when the United Website says that X (economy reward class) seats are available, I'm finding it a bit hard to believe what Virgin are saying.

Is there some trick to searching myself or a particular booking class which these flights use?