The sole reason for cannabis plants not to flower is that they might be photoperiod plants and the light cycle may not have been properly changed or changed at all. When growing indoors, remember that marijuana plants are to be exposed to a light cycle of 18 hours of light / 6 hours of darkness during the vegetative phase, that is, during the growth phase. That cycle must be switched to 12/12 once we decide the time to flower has come. Unless you change the light cycle, the plant will not flower and will remain in a vegetative stage ad eternum.

Outdoors, the flowering phase begins when days start to become shorter, by the end of summer-early autumn. If your plant hasn't flowered by the time that moment arrives, it might be due to the light pollution of the night sky caused maybe by a nearby lamppost. In that case, we recommend covering the plant at night, with a dark mesh or plastic –always letting the plant breathe- that prevents direct sunlight from reaching her.