I am having a similar problem with my Gold Sex Links. Their feathers all look bad and they are missing quite a few in their rear area (except for the rooster). My rooster NEVER loses feathers or looks poor, but my girls all look bad. Even my young pullets that are about 15 weeks old look the same. But I have never seen a louse or a mite on them. I keep Diatomaceous Earth out for them to dust in thinking that will help.
I DID get a tip from a friend who raises chickens that I have just started this week. She suggested I put 1/4 cup of Organic Unfiltered Apple Cider Vinegar per 1 gallon of water out for them everyday. She said it makes a HUGE difference in their feathers and their overall health. I began this on Monday. So far, I THINK they may look a little better, but I can't really tell a big difference yet. I am going to keep it up for a few weeks and see.
Anyway, I also question whether you can see lice and mites or not? I've never seen anything moving or crawing on them or on their feathers. They do have a lot of something that looks white and flaky - like a white feather "dust" or something. But it definitely is not alive whatever I'm seeing! And it's not the Diatomaceous Earth, either. It's more flaky than that.