As Mr Sail said, it often takes years before 2 large parrots bond. Parrots are not like dogs, who just breed when they meet the first time. Parrots will only breed with a mate they have bonded to.
The second thing to consider is cockatoos are a little special, when it comes to breeding. The male cockatoos are a bit more aggressive towards their mates than other parrots. It is not uncommon for a male cockatoo to kill or severly injure the female cockatoo. It is not more than a few weeks, where a friend of mine had to put down a pair of tritons as the male had maimed the female to a point to where, she was beyond saving and for that reason, they put the male down too.
The third thing to consider is, that you shouldn't split up a bonded pair of parrots, as this usually makes them so unhappy, they often start to pluck. My male scarlet is a severe plucker (bare on his entire body) due to a loss of mate 10 years ago. He has had a mate since, but has not stopped plucking.