Desperate for help

I have had a long term issue and am at my wit's end...a year ago, I had a 18 month old EE develop some bald spots. There was NO evidence of pests. Everyone said it was a slow molt. At that time, i treated with ivermectin to be on the safe side. However, the loss has slowly but surely continued. There is still no evidence in the coop (have checked multiple times at different hours of night) or on her of pests. They are not pecking each other. I do not have a rooster. They are still active, eating, drinking and laying, although laying has slowed considerably in the last couple weeks, which could also be due to loss of sunlight.

I wanted to be safe again, and so decided to retreat all the birds. I dosed with ivermectin, used Sevin and DE in the coop two weeks ago (after cleaning). Yesterday, a different bird had major feather loss. I at first thought a predator had gotten a piece of her. However, Her skin and feathers looked fine, except as I was examining her, feathers would fall out in certain spots at the touch. I repeated the ivermectin and Sevin coop treatment yesterday night. However, today I noticed my three oldest birds doing a lot of preening. I'm super concerned and have no idea what to try next.

For reference, I have 2 EE's 2 1/2 yrs old, 1 6 month cream Brabanter, and 3 3 month buff Orpingtons. For the past month I have had them on a 20% protein food, organic and no soy. I would greatly appreciate help!!!!!!