Some of my girls have had this off and on for a couple years. I'm not exactly sure of the cause although some berries from a tree seem to be part of the problem. However, I accidentally discovered something that is helping them immensely! I started using up some green probiotic powder I had on...
What about the idea that they are missing something in their diet (I.e. Needing more protein and fat) that's in the eggs? So perhaps supplement with some high protein and fat foods for a while? Beans, hemp, etc