Parakeet won't go in the nesting box?

The female is a year and a couple of months and the male is about 1 year old. They are a bonded pair. They regurgitate, preen each other, eat together,ect. I also have another budgie in a separate cage so they hear other birds chirping too. I added the nest box and it has been on the outside of the cage for a couple of days. The only thing I noticed is that once in a while she puts her head inside the hole and looks inside. But it is becoming less now. They still regurgitate and preen all the time, and they are actually regurgitating more. They also chew on the mineral block and cuttlebone a lot. Sometimes the male will do a courting dance with the female. The females cere was just white but now i am noticing it is turning kinda brownish. It is a light brown now. But why wont she go in the nesting box. If she interacts at all with it all she will do is poke her head inside for 2 seconds. Why won't she go in the nesting box?