Is this really biobizz lightmix=? lookstotally different, as if you mixed it with peatmoss. Your seedling Looks concerning. Normally you have no Problem at all after planting it into a soilmix like that.
iam a Little concerned About the Color and the look of your seedling.
ok lets do it like this, i talk About good conditions and you may see what went wrong.
After Germination, automatic Plants love afinal pot, filled with afluffy Mj-growing Soilmix like yours.
ThenJust alittle water every day(20 ml) and if you wnat to afoliage sparaying with Aloe or aroot enhancer
Not to close to the Light, and NO FEEDING... for the next weeks ( mostly around flowering time)
ok....
as you are already in week 3 and ist an automatic, i personally have no hope for a big Plant.
You can let her grow along
But you may start new, and giving her the conditions i have written dow,
Additional you can give the Seedling more RH in Giving them " open domes"
that means, you cut Plasticbottle in half, poke holes in it and turn it over the Little one