I'm not familiar with that soil or nutrients. Maybe others can comment. (Or, you could ask in the general->nutrients forum what people normally feed).
If you think you're underfeeding, then I'm inclined to say it's Mg def. I would add 3/4 tsp/gal epsom salt to the next feeding. (That's about 2ml/L, I think).
Do you ph your nutrient mix? (I don't ph my soil. I don't think you have to. But, just curious if it's possible your ph is off.). Do you have an EC meter? (What's the EC of your water? And, the EC of your runoff? That would be useful info.).