Need help..my first grow using nft system

this is my first grow i ve had my plants under 4x 600w lights in nft systems for about 6 weeks now and they dont seem to be growing much and the little ones aint grown at all.im using ionic hydro grow nutrient at 1.5 ec. water ph 5.5. where am i growing wrong??

first off need a little more info......PPM...solution temps and how many times a day your pumps come on............how much food are you feeding it............1st i'd go to get a three way water tester for NPK......you can get it at any garden place.......it'll tell your NPK content individually..............then i'd flush the system.....wait a day or so of pure water thru system.........them i'd start feeding again............gotta have nutes per/gal...........cal/mag, if you don't have any get some epsom salts..............very seldom have a cal defic without some kinda lockout goin on................lower the ph to 5-2-5.3..............keep the solution tank out of the grow room.......if your gonna use rockwool......i'd keep the pumps down to min...........shit stays wet a long time...............