I don't know but I have ideas that could be considered.
We are not there so we have to rely on the photos for inspiration and what I see is an overwatering and underwatering situation happening at the same time. Follow that up with not enough light getting to the seedling and the combination can slow down if not actually stop growth.
Overwatered because what I see in the photos is a circle of approximately 4 inches (100 mm) of saturated soil mix. The surface of the soil within that circle looks like it is so wet that it is sparkling where the light is reflecting off the water. Then the underwatering shows with the soil between that circle of wet soil and the edges of the containers; looks dry, very dry, almost as if the dry soil goes down several inches, maybe 2 or 3 (50 - 75 mm).
What is there now is a small ball of water saturated soil which can cause the root system to grow slowly while it is surrounded by soil that might be dry enough that the feeder/hair roots will not grow into it.
Not enough light getting to the plant will then slow down photosynthesis and that slows down the leaf and stem growth and then slows down the root growth. I can see the condensation on the inside of those plastic bottle bottoms caused by the trapped higher humidity from the wet soil. All those drops of water are blocking enough of the light from getting through to the leaves so little, if any, photosynthesis.
My suggestion is to remove the plastic bottle parts which lets the soil around the plants start drying out. At the same time slowly give small amounts of water to the soil further out from the seedling. Not a lot at one time but you want to feel assured that the soil in that area is moist but not wet. You want it moist enough that you can feel assured that if the roots grow into that area they will remain healthy. Once it gets figured out the soil will remain moist for several days at a time while the very surface for a few mm down will be dry. No need to mist the plant or the soil. A small watering can or even one of the plastic bottle bottoms will work and reduces the possibility of pouring too much on at once.
It is not an overnight project. It will take several days to balance the soil moisture and if all goes right then a few more after that for the plants to react.