I use co2, ph every feeding, temps stay in the high 70's low 80's. My plants can be 10 inches from the light without stress. I have a 620 cfm fan and a thermal heat shield on the reflector(1kHPS). I water whenever the container feels light or the surface dirt is completely dry. I usually get about 1 oz per plant if vegged to 12"-18".
do you use a co2 regulator and monitor to keep 1500ppm co2 during day and off at night? do you run a closed system or do you have an exhaust fan running constantly? co2 is an excellent addition to a system that is already perfected but if you have any other part of your grow that is bottlenecking everything then you won't really see the advantages to using co2. to clarify, you'll only see the benefits to co2 if other aspects of your grow are flawless.
what kind of soil are you growing in? what kind of nutes are you using? are your plants showing any sign of nute burn (burnt tips or a solid green stem?). if you overfeed a plant it will never be able to express it's full potential.
what kind of strain are you growing? is it a known high yielder? if you think you are doing everything right then you could just have a low yielding strain.
answer those questions and we'll go from there...