Forever molting

I'd suspect external parasites and give them all a good dusting with a permethrin poultry dust, 2 treatments about 10 day apart.
DE won't get rid of an infestation, it might help prevent one.

Go into the coop well after full dark and wipe the bottoms of the roosts with white paper towel, look for red streaks of squished well fed mites.
Then inspect the birds by parting the feathers right down to the skin around vent, head, neck. and under wings.
Google images of lice and mites before the inspection.

It might also be feather mites, which you will not be able to see as they live inside the feather shafts.

Thanks. I finally called the vet. He said worst case of molt he'd seen. No mites or bugs and feathers are trying to come back. Birds are also apparently older than I was told and are in their 4 th year. I'm gonna get that dust too cause of the feather issue

Might be a good idea to dust anyway....and get them on a higher protein feed without calcium, will help grow feathers out.

I like to feed a flock raiser/grower/finisher 20% protein crumble to all ages and genders, as non-layers(chicks, males and molting birds) do not need the extra calcium that is in layer feed and chicks and molters can use the extra protein. Makes life much simpler to store and distribute one type of chow that everyone can eat. I do grind up the crumbles (in the blender) for the chicks for the first week or so.

The higher protein crumble also offsets the 8% protein scratch grains and other kitchen/garden scraps I like to offer. I adjust the amounts of other feeds to get the protein levels desired with varying situations.

Calcium should be available at all times for the layers, I use oyster shell mixed with rinsed, dried, crushed chicken egg shells in a separate container.

Animal protein (mealworms, a little cheese - beware the salt content, meat scraps) is provided during molting and if I see any feather eating.