Plants growing slowly

Hello everyone.. Ive been growing 16 kush plants in veg for about 4 weeks now and they dont seem to be growing as fast as i would have thought.. This is my first grow by the way.. But from wat ive read they shud be at least a foot tall.. About half are between 15-20cm and the other half are 20-25cm... They were under a 600w hps light for two weeks while they were in 1litre pots and have been under two 600w hps lights since i put them in 5 litre pots two weeks ago.. They get fresh air and biobizz grow nutrient.. Wat could have caused them to grow this slowly or am i paranoid?

Not sure but I would assume that you're better off with MH which is more on the blue side which is best for grow HPS are better for flowering but can be used to veg but not as efficient...that's my opinion.

They are duaL spectrum bulbs.. I was told at the hydroponics shop that they have 30% more blue light than standard hps bulbs and work well both in veg and flower

watering too much maybe

something ain't right. When I saw your banner I was sure I would find that you TP'd directly into a huge 5-gallon pot.
That will cause you to think there is no growth because all the action is below ground. As soon as seedlings get into dirt
they send out roots to feed and stabilize the plant. Until the first roots hit the inside of the pot there is little growth above ground.

One other thing that comes to mind is your soil. Years ago I was growing a tiny auto with some real fine soil. Come to find out
my little auto wasn't able to spread her roots out because the soil was too compacted.

Good luck, BigSteve.