What size planters?
How close is the light?
Do you have good airflow- including intake/exhaust?
How do you water & feed?
We grow it as well- and let me tell you- they respond BEST to LST...
if you FIM you are creating extra branches on an already branchy plant.. DON'T!
Just let her go until she has 6-8 sets of branches and start tying them down....
Use a BIG planter...
they should be in a 5-7 gallon MINIMUM as these girls like a longer veg time.
Be sure the lights are cooled and down as close to the plants as possible.
Feed only 1x a week mixed according to a feeding chart, and giving them enough solution to totally saturate the soil and get a little bit of runoff out the bottom of the planters... DO NOT water unless needed between feedings- meaning the top 2 inches are dried and the planter is light....
Did you count the days to harvest correctly? It doesn't start day 1 the day they go into 12/12-- they take a while to start budding-- so the day they actually start to actively bloom is day 1... so you may be cutting them early.
Just saying-- so many things effect the yield-- we just "bottomed out" on a grow because of the planters-- too tall, nit wide enough, and the drainage holes not on the bottom but about 1/2 inch up--- so instead of saturating the soil- it was running down the sides of the planter, most of it going out the holes and some pooling at the bottom... forming a solid brick of soil 6" deep at the bottom of the planter.... the roots couldn't penetrate... so the plants were stunted at week 5 of flower... never got any bigger...
Check it all!!!