pics? so 3 wks ago u switched ur lights to 12-12...okay and she is showing no flowers or no pre-flowers or both? some plants can take 2 weeks to start showing pre-flowers & that is quite normal, other longer flowering sativas could take 3-4 weeks. More info would be needed to help u to answer that question. As well, of course, ur lighting is a high priority factor on the flowering process & if ur lighting is weak or insufficient, it could be another month before u start seeing flowers.