Possibly overwatering...? Spraying too often...? Insufficient air circulation?
Definatelly too young to need nutes.
Is that desk lamp a growlight?

Think I remember stinky (are we supposed to capitalize her name, or leave it like we see it...?) mentioning Fafards, but not sure if that was really it. The soil looks pretty good in the pix, tho.
But, since it has nutes already, likely they won't need any additional veg. nutes for a month or so. (give or take a week or so) Some folks use Superthrive or a micronute suppliment. I'm among those that do so, but with caution not to over-do it.

A seedling mix may offer more consistent results, if the Fafard's is too 'hot' for your seedlings or clones.

How did the roots look on the plants you did transplant? Rootbound, or just a few on the sides? Likely leaving the one in the cup would be ok, as transplanting a distressed plant is not optimal. Unless the soil is the problem, lol. Will have to transplant eventually, tho. All will need bigger pots soon. Any plans to get more of the same soil, or are ya trying to wait till you see results?