how often are you watering and for how long?
I would say it was a watering issue as rockwool is tricky. Overwatering or underwatering will kill them off quicker than a bit of a low PH.
5.5 for rockwool is fine for a while. But staying soggy will kill them off!
You still have some issues but its def coming around.
I woukd stick to hydroton or another soiless medium if your just getting into hydro.
Seen ALOT of folks start in R/w ..get discouraged and go back to soil. Hang in there and dial in your watering times and ph/ppm levels.
research research research!