plants do not grow

hi guys. my problem is as follows.
germinate 20 seeds of super critical, germination result 100%
i transfer them in vermerculite and after 5 days, all came to a height of about 5 cm.
then they seems to stop growing and they did not gain in height. now the plants are 2 months old
and they are like small bonzai plants. they are healthy and none of them died so far, about 10cm tall.

i repeated the growth using some mixed flower seeds. perfect growth for all and got flowers after 40 days.

i now put some more mixed marijuana seeds ( super critical , afgan, ak ) but am getting the same results as my first attempt, am getting the bonzai effect again.

i use a 400w hps, cooler, extracter, for 1 sq meter. the temp in the room is about 27-30 degrees celsius.

can anyone help me ?? or explain why did i get 100% results from the flowers but getting bonzai from marijuana seeds.
thanks.

Excuse my ignorance, but are your plants auto-flower? If not, you immediately need to get some MH lamps...soonest, now, not next week.

Veg, I reccomend a 3:1 of MH to HPS, flower would be the other way around.

Your post is somewhat confusing.

Have you actually flowered any of the cannabis yet by switching the lights to 12/12? This is when they stretch and get tall. When you veg them - light on between 18 and 24 hours a day - then if there is sufficient intensity of light the plants will stay pretty short depending on the strain. The strains you mention are mainly Indica doms, and vegging them under 400w HID would likely result in tight nodal spacing and therefore short, bonzai like plants. This is desirable.

If on the other hand you have still got short little squat plants whilst flowering, then maybe it's unlucky phenos, too short a veg time - things like that.

don't know wut the problem is there but I don't think its the need of a MH...I had a 250w MH/hps that I used to veg, the MH blew out & I started using my hps & never skipped a beat & also never bought another MH bulb & its been years...I start with cfl's for a couple weeks then in she goes

Its sound like a PH problem. The type of light you are using in veg doesn't really matter. Pictures and more info would help you more.