8 week old SLW - 2 or 3 are quite bare

I have 8 SLW chicks. For several weeks, 3 of them are experiencing varying degrees of bareness. One of them is very bad - has an almost completely bare hind end. The skin looms pink too. The other 2 who are affected are not as bad. The rest are feathered in nicely. I cannot see any signs of bugs crawling on them. I have also spent significant time observing and do not see a pecking problem, although they are part of a flock with older chickens. They are in a very large electric fenced area and have lots of room to seek sun or shade and to get away from each other is case of pecking.\\

Any ideas?