Why is my yield so low?

Two things jump out at me: air circulation and ppm. Your ppm sounds too high, in my experience better to underfeed than overfeed. I didn't do the math on your room but approximately 1000 cfm seems a little low. In this case better to over exchange room than under exchange. I'd back off nute levels and increase air flow. I have the same exact strain from Harborside and get 3 oz per plant with 2-3 wks veg. I'm in sunshine #4 which is basically hydro. I Also do rockwool in ebb and flow but haven't done the atomic in that yet. I run 1000 watts over them. When i do rockwool with master kush never exceed 4ml/liter advanced jungle juice which should be about 1400 ppm. The Master Kush is a relatively high feeder too. On another note i had major yield issues earlier that i thought was disease or nute lockout but turned out to be Broad Mites. If you live in bay area i'd check to make sure you don't have em. They are microscopic and you'd never know it until the damage is done.

The obvious reason that you are yielding so low is because of the rockwool. Not only is it EXTREMELY hard to maintain optimal conditions, but root space is very limited, thus limiting the plants growth.