Looking dark green there. When do you cut-back on nitrogen? It's getting pretty deep into flowering, so you might want to think about restricting the nitrogen, or flushing it out of the pot.
Doesn't look like major ph issue, but I see a few leaves that make me mention it.

Likely best to fill-out the troubleshooting form.

From seeds or clones? All the same strain? Have you grown this strain without incident before...?
If clones, how long had they been rooted before switching to 12/12, and how tall were they when you switched?