Challenges with new grow

Previous successful grow from assistance on this site, but starting to think was beginner’s luck. Here is the rundown on these:

Germinated using paper towel method without issue.
Using Grodan soaked in pH’ed water at 5.8 with moisture dome.
Light was initially a 27w grow light for starters. Cycle is 18/6
Fan is on in room to circulate air
Nothing but water, 5mL at first, second time 20mL each. Final feeding was 20mL of about 75ppm.
Using Grodan and Hydroton-like medium to eventually end up in DWC.
3 of these have a root showing at the bottom of the cube, but others seem “stuck”.
The youngest ones are 2 weeks young, the others are about 1 month!!!
I’m doing something wrong, I just can’t seem to see it. Assistance appreciated!

Looks like u may need to feed, they look hungry.