Stunted Height But Plenty of growth?

Hi ive just potted 3 diffrent types of plants in 8 litre pots with coco canna 1 of these seems to growing normaly while the other 2 are seem to have stopped at about 8" high , What i cant work out is why there is plenty of growth on these 2 and they look really healthy, but i cant get them to grow any taller ,ive been veg them for 6wks now. with 16 hrs light on. Ive never had this problem before and i have done a few plants now , can anyone give me some suggestions . (pics to follow )

Im using a :
400 watt
canna cocco
A and B nuits

strain, stress etc. Pics are gold in this case.

minumun 18 hours light for veg! ive seen 20 22 and 24 but under 18 is not prime for growth. not sure if this is your problem but i would fix that first.