Why would I give that advice if it would hurt your plants? If you have a tough strain that refuses to enter flower or one in flower that seems to never ripen for harvest, reduce the number of hours of light. Not all strains do well with 12 hours of light in flower. Equatorial strains may require 13-14 hours, whereas other strains may do better with 10-12. 12 hours of light for cycle is not science, it is an estimate of what works for the majority, the middle.
If after 3 full weeks of 12/12 and you have zero evidence of sex, reduce the number of hours of light. Go 11 if ya wish, instead of 10... If your plants started their stretch under 12/12 and have stopped without showing sex, the plant is getting too much "sun"
I know this is an old thread, but the problems seem to recognize no time era. Some of you may know me and my unique problem I had/have with one of my plants. A white Widow autoflower that I topped before looking it up to see if Autoflowers
could be topped and then the next day I almost sucked it up into a shopvac.
Even though, surprisingly, it did not go up the tube, all the leaves were just hanging as if dead. It was only a week old when I did it. For days it did nothing, and then it started new growth. All the abused fan leaves did die. So I cut them off and prayed for the best. After repotting it in Royal Gold Tuper it really thrived. It become UBER healthy with magnificent foliage. But it won't flower. It got to 18" and I decided to force it. I have it under 12/12 for 4 days now and no little hairs or ballsacs. Today while I was inspecting it with a magnifier, I got it too close to the lamps and burned the top fan leaves.
So I would bet that didn't help anything. I have it on the same nutes as the rest that are doing magnificently.
Is it possible that it may never flower? It is truly a miracle that it is even alive much less so hardy.