When my girls first started laying, we had eggs with very strong membranes around the yolks. In fact, if I wanted scrambled eggs I had to grab the yolk and break it with my hands because they wouldn't smash with a fork. Well the last month or so they are very weak. I can't flip a fried egg without breaking the yolk. Is there something missing from their diet that causes this? They are fed layer pellets and oyster shell, plus table scraps now and then.