Rock, I'm suspecting overwet soil if you just had a little bit of root development near the soil surface.

If the soil doesn't drain well, there's no drain holes in the pots, or you just have kept them constantly saturated, the roots would not have had sufficient oxygen to develop.

Soil or grow media should be allowed to lose about 3/4 of its water weight before watering again. Soil should be touch dry down a couple inches before giving more water. Think 'damp' never 'wet.' Cannabis hates 'wet feet.'