Well-at least we know that they aren't just hiding them from you. You can up the protein by feeding things like meat. I use fishing worms myself. I know some people worry that chickens will get worms from them, but I don't worry. They find them in the ground anyway. I am always watching for...
I have a couple questions for you.
Are they free ranged at all? It they are, they may have laid them somewhere were you can't find them.
Are they getting enough protien??? That's a big thing.
And are certain of their age?