Rdwc stunted plant what’s wrong?

[Ericlch]

I’m running a 40 gal rdwc setup, 3 of my plants seem to be doing fine just 1 have slow growth and leaves seem to have light green spot, they on week 3 of veg.
Ppm is 340
Ph 5.5 - 5.8
Water temp 68
Roots seem to be doing fine.

 

[TripsRabbit]

I'd lose the mylar, it creates hot spots and can cause light burn in spots. White is better.