What kind of light are you running ? I had some of the same problem. The tips don't look like nute burn, so either a light could be to close like HPS or MH light. Which can cause the leaf to curl trying to save water, low humidity plays a part in that. Or you need some calmag plus. I used a small amout of epson salt till my calmag came. Are all of them affected ? out of six only 3 or 4 were doing it on mine, all are northern lights. Hope this helps , this is what I found out just not to long ago. Calmag is calcium magnesium. Epson salt is magnesium salt. hang in there. Maybe one of the big guy's can help? When they start to sex you will know, white hair or balls. males show first.