Are your plants close to the light? Like someone metioned above, it could be that the tops are too close to the HPS and are heat stressed. Also could be a nute problem. I think those are your two best solutions. I would think the extract you are using is safe as long as you are using it as directed. I like to use a little less than the recommended dose when using a new product for the first time. Just keep in mind 95% of crap on the market is just money making snake oil that really doesn't work. I'm not saying what you have is crap, but that is from what I have noticed by using many products in the past. Just research and see what others have experienced. Good luck!

TGT