Sometimes you'll get a brood of chicks and notice that while many have grown in most of their feathers by eight weeks or so, one or two are slow feathering and seem delayed. It can be concerning, especially for first-time chicken moms and dads!
It's usually nothing to be concerned about. Genetically, some breeds feather out more slowly than others. Breeds/crosses used for meat in particular tend to have slow-growing feathers, along with Brahmas, Cochins, Cornish, Rocks, and Orpingtons. In addition, breeds or crosses designed to be feather-sexed often have slow-feathering, so you may see it in sex-links like black or red stars.
Slow feathering can be an issue if your bird is not getting fed well or is getting fed mostly treats such as scratch. They need good nutrition and lots of protein to grow feathers. But if your bird seems otherwise healthy, if your flock are being fed well and aren't picking at each other, then slow-growing feathers are nothing to be concerned about.