I had one dominant sativa just go into flower about 2wks ago, but all my other strains have been flowering for awhile, basicly about 5wks into flower for the rest. Im assuming you live in the northern hemisphere, one thing you should do is to feed the plants some nutrients that have a much higher ratio of phosphorous and potassium something like (1-5-3) ratio would work fine, your plants need these added amounts of macro nutes to help the transition. Assuming you are in the N. Hem. theres no reason that I can think of that your plant shouldnt be flowering, we've had the necessary sun light cycle for over a month now, plants dont take exactly 12/12 to flower, Im forgetting at the moment but right now during flower the plants are getting just over 13hrs of light I believe, maybe someone knows something I dont, good luck, peace!