Why does my Vine Bougainvillea only grows on short days..

yes, to all your questions..fertilized, re-potted it, over 6 hours of sun. I know the differences of the zones, that is why in my OP, i mentioned NYC and the short days. I brought in the vine with still some bracts in color because Fall was approaching and for what it did not do during the summer, it started to grow continously in a south exposure window. ..but i was hoping to get all that during the summer and outdoors.

I'm aware of different Zones, that is why I mentioned NYC. I bought 2 plants last August, so for me, thats new. They were both in full bloom then, but of course, they came from California. In a couple of weeks as Fall was approaching, the blooms eventually stopped and I had to bring them indoors in a south window exposure. There they started to bloom again and the one that is a vine, started to show that growth. I nipped it off, cause i wanted a bushier plant...anyhow this continued till Novemeber and nothing after that. That is why I thought these are considered short day plants, like the christmas cactus and etc. i read about the epsom salt theory, but wondering if its too late to try it out now, when Fall appears in a month from now.