i hate to disagree with the people who say that it maybe a long flowering strain.. all long flowering means is that it will flower for a longer amount of time, not that it will take longer for it to start flowering.. and i[m pretty sure that northern lights is an indica dom strain, which probably has about an 8 week or so flowering period..
i would remove that light, like you said you did, and give them like a week or so, and see if they are showing any more signs of flowering..
g'l with the rest of the grow m8..