To me, it looks like it could be a bit warm in the space.. Also, these look like a longer variety sativa and that they could go a ways longer. 15 weeks 12 x 12 on your lighting? Have you looked at the trichomes to see how they're maturing? I run my lights on for 11:45 to compensate for any difference in the timers. You could try taking your lighting time down to say 11.5 hours on, which may speed up the finish a bit...

Peace, Farmer Rich