2weeks and no grow

ok guys basicly i have cuttings that are growing 4 are of the same strain and 2 are very diff.

im growing them in soil and feed them as normal and the 4diff strain plants are fine and growing.but the other 2 plants refuse to grow.they have been same size now for 2weeks while the others have grown abt 8inch..

can anyone tell me the problem or guess at what it could be..
heres a pic of the plants

the plants are under a 400watt hps light. all reflected around with mylar.... they receive plent of light and are fed twice per day.They all get the same ammount of water so i know im not underwatering the plants as the other 4plants are doin great

it looks to me a little underwatered....and so others could be of more help some more info on your set-up would help.

6 plants under 1 400watt HPS light. as there only small plants they all receive alot of light.i feed them 200ml twice a day of coco A & B nutes... i water each plant the exact same ammount everyday. as i say the other strain is fine all 4 growing well.just the 2 diff strains are not growing