stems pinched and falling over

Beautiful Blue Mystic, about 3 weeks into veg on 24hours of CFL 200 W. Plant looks awesome, but today I noticed the stem was narrowed and the plant was listing. Happened on the last Blue Mystic grow and I lost that plant. I just buried this one deeper in the coco medium.
Both grows were hydro, last time in hygroton (clay balls, can't remember how to spell). this on in coconut medium. Both times started Northern Lights concurrently, with no issues, those plants are either smoked or just went to bud...
Any idea why a stem would narrow, turn brown and the plant would fall over? I also only got 2 of the 5 Blue Mystic seeds to sprout? Could it be a bad bunch of seeds?
 
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