Is this a nutrient deficiency?

This one is actually about 4 weeks old, and is still tiny! The other one is doing pretty good at the moment, but this one seems lacking.
Growing under fluro in seed and cutting potting mix (no slow release rubbish, just soft loamy stuff). Soil ph is around the 7.5 mark on my crappy soil ph kit, water is rain water, but around the 6.4 mark on my aquarium ph kit.
I have given this one a 1/4 mix nutes about a week or so ago, which I think has made little impact. The redness appearance was there prior to nutes. I was hoping they would have helped!
This and the one in the background are essentially raised identically, but this one is struggling. It's hard to take quality pics with detail on such a small planet.
Fire away with questions and comments!

Feed a 1/2 strength feeding. Don't water again until it actually starts to droop.

4 weeks old, not good.
What are you using for grow medium?

As I did mention Growing under fluro in seed and cutting potting mix (no slow release rubbish, just soft loamy stuff.

I'm guessing since its seed&cutting potting mix it has very light nutrients in it? The soil looks kinda like it has chips in it and maybe those are specs of vermiculite around it? I know alot of seedling&cutting soils have peat moss in them and small wood chips/tiny sticks. I think they do that to hold the moisture in and keep the soil moist longer because cuttings and sprouts like a certain humidity and moisture level. It would probably do the plants some good whenever you transport them to put them in more airy soil, maybe try adding 25-30% perlite into your soil mixture.

*UPDATE*
I have transplanted the plant from the seed & cutting mix into vermiculite, perlite and peat moss mixture. I think I will give her 7 days, no improvement and I might cut her down. She should almost be going into flower now, not just getting the 3rd set of leaves!
Any questions and comments?
Fire away!