I can't get my parakeets to mate! Any help?

I have a nesting box. 3 males and 3 females. 1 pair seems to LOVE each over. 2 of the females ceres are deep brown and all 3 of the males have a deep blue color on their ceres. We just got a HUGE cage. They seem so happy. I put the cuttlebone near the box and even some spray millet too. The male always follows the female and the rest just sit alone. (Im waiting until my young parakeets grow up too. Ones a female ones a male and i hope those 2 mate) Then i have the 2 others who love each other but dont spend enough time together. ANy help?