My first thought would be lack of water. For that matter, overwatering can kill roots and have the same effect. Really not obvious from your pictures, but is the soil moist? Is there good drainage at the bottom of the cups to avoid pooling? In fact, what's in your soil? Going to be hard to diagnose unless we know more about your setup.

Have you recently transplanted them? If so, was the soil very dry when you did it?

Thank you for guiding me. Your guess is correct. I was afraid of watering too much and the soil was not wet enough.

Also, I transplanted them to the new pot and I just wetted the new soil

There ya go. They will perk up after a good soak!

It's always tempting to use potting mix straight from the bag, but it's often too dry. I usually fill up my pots and then pour boiling water over them because it absorbs faster as well as sterilizing the mix. Of course it is necessary to wait until it cools down before transplanting seedlings into the new mix.

I always heat-sterilize my mix. Never thought about using boiling water to do it, but yes, hot water wets soil better than cold water does. I have to suspect that boiling water doesn't do a very good job of sterilization, though, because by the time it gets to the bottom of the mix it isn't boiling anymore.

With regard to soil moisture, good to give some thought to how to assess that. If you dig down an inch, and it is not moist, you need to add more water. What you want to avoid is sopping wet soil, where you can actually see droplets of water. But if your drainage is good, it is unlikely that will happen.