I would think they are still devloping roots. They should spring up soon. They look healthy and happy to me! This week is a good time to start feeding them 1/4 strength nutrients and build up from then.
I would also bump one out and check the roots it is possible they grew roots fast and are root bound and ready for transplant. I am not a pro but thats what I would do. They look healthy so thats a great sign lol.
Pataince my friend is the best thing people can do for their babies