It's either that the plants are really stressed and taking time to get over it or you could have the beginning of rot. Obviously they weren't the best condition going in and if there is any doubt in your mind that it could be rot I'd remove the plants since they share the tank with the others which seem healthy.
also when I do DWC I like to start the water level high like you but once the roots grow into the water I let the level drop to give about 2 inches of an air-gap which helps prevent rot as well.
At this point in time just keep an eye on them and it wouldn't hurt to run something in there like a ~zyme product or h202. Also many people use bleach, I have never and can't give any advice on that but you could search for info on it.