Which ones are you referring to? The ones in the middle look fine. How many weeks have they been growing? The chard off to the sides doesn't look so good. Were these planted at the same time? What are your pH and EC levels?

This is my swiss chard after about 6 weeks as a reference. You can see I've already harvested some outside leaves. That's the great thing about chard, you don't have to wait until it's full grown to enjoy it.

Mine is about 8 weeks and I have not harvested any. All plants were put in at the same time and dont look anywhere as healthy as yours. However I must admit I did clean out the 54litre reservoir and put some new fresh nutrients in from another supplier and in the last couple of days they seem to be perking up a bit. Thanks for sharing your swiss chard information and photo.

My PH is 6 and my EC is 1.8

Are your plants growing indoors or outdoors? mine are growing outside under my deck but open to sunlight and wind which shows in my photo as the plants keep leaning with the wind.

You pH and EC is fine. Since they're are perking up with a nutrient change it may have been lacking in something. Chard is not really a big drinker of nutrients though. Mine drink a lot of water and EC levels drop slightly each day but nowhere near something like a tomato plant but depending on how often you do a water change there could have been some deficiency. I'm assuming your prior nutrients were a complete mix and more of a vegetative mix in regard to NPK. Although 8 weeks with that little growth on some of those is odd.

I'm sure those roots of the smaller ones aren't very developed in comparison to the others. Where is you water level in relation to the bottom of the net pots? The bigger plants probably have plenty of roots down into the nutes but the smaller ones, depending on your water level and how much agitation you have in the res with airstones, might be starving a bit. I have found that even an inch difference in water level especially with younger plants can make a big difference in early growth.

Tough to tell from the photo but are the "runts" being shaded at all from the other plants? I have one plant out of seven in my bucket that is half the size of the others and it got shaded early on from some other leaves since I have those plants packed in there and now it's the runt. Although sometimes one or two plants just lag the rest for seemingly no reason. Kind of like when you plant a bunch of seeds and you weed out the weaklings before transplant. Same lights, same mix, same seeds, yet not the same growth. Anyway, I'm rambling.

My plants are inside under T5 lights along with some other stuff.