Their age (they need to be at least 1 year old by the way)
How well they have bonded.
What their environment is like.
I also read that their peak mating season is between October and March... I assume they can still mate outside that though. Also, having 2 or 3 couples sometimes helps, as they "set-off" each other in terms of breeding apparently... maybe you could organize a 'play-date' with another parakeet owner?