The only thing I can add is; be patient! I know it's hard, but that's really all there is to do at this point.

Sounds like you've done everything right, so it's probably just a slower growing strain. UNLESS, it's by some odd chance an autoflowering strain (which I've heard won't flower under 12/12, but that's just what I've heard - I have no direct experience in this).

So yeah, I know, probably no so helpful.