Slow to Flower

Why would one of two cannabis plants that I am cultivating be so slow to show its sex in flowering. They are of the same strain , same condition's ect. One is showing her feminine traits while the other shows no signs. Im 3 weeks into the flower stage?

Possible light leak. Its essential that no light gets in your room during the time the lights are out. Could just be that plants genetics. But I'd definitely make sure you don't have any light leaks.

alright...Im working on that! Thanks