I'm assuming you have the two budgies in an individual breeding cage with a nest box with a concave bottom and are giving them egg food, calcium supplements, and vegetables in addition to their high quality seed mix and pellets.
Abundant, high energy foods will encourage breeding behavior.
Make sure the budgies are getting plenty of daylight -- a good 12 hours of light is important to stimulate breeding instincts.
Make sure your female has shredding materials and soft wood to chew.
Be sure you give them some privacy as budgies often prefer to mate when they aren't being watched.