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"