After weeks of struggling I did finally find out what was the problem with my girls. Their roots were not well developed due to the rock wool wall removal and the early transplant to the final pot.
For this reason I bought the Rapidstart by General Hydroponics to boost the roots and I worked out pretty good.
I also stopped the high fertigation to stimulate the root growth. Yes, I realize now that high fertigation is suited when the plants have a good root system.
Anyway, they are thriving now.