Maybe try and make the dark time longer by an hour or so. Go to 14 dark, 10 light and the plant should get the hint that it should be flowering.

I read somewhere that sativa strains can be like that cuz they are from near the equator where the length of days changes the least. The longest day of the year at the equator is much shorter than the longest day in the summer up north. 12/12 is almost normal natural veg lighting that plants get at the equator.

From wikipedia: Day length - Wikipedia, the free encyclopedia
"On the Equator, the length of day is not exactly 12 hours all the year round, but rather â?? due to atmospheric refraction and the size of the Sun â?? exceeds 12 hours by about 7 minutes each day;"

So, yea, I'd make sure you have no light leaks and increase the dark time by an hour or two if you're having reveg probs, especially with sativa strains. Something to try anyway without having to chop anything down.