Give them a once over - if they've picked up any parasites, that will run them down a bit and interfere with lay (like bird mites or lice). Heat will affect them - I get less eggs in the heat of summer as its really hot here. Also, look for any possibility of predation. I had ducks laying 12 or so eggs a day one spring and then NO eggs for several days! I paid attention and realized that a Raven was flying in the open top of my pens and stealing all my eggs! We put up bird netting and boy was the Raven frustrated the next day. However, I got my eggs back. Look for any holes where a rat might be taking them or a snake might get in (though snakes will usually leave crumpled up egg shells behind at least sometimes). You can add additional protein as well. Try liver, either chopped and raw, or cooked - mine won't eat it raw, but will if I cook it and run a bit through the food processor with some rolled oats. I get liver free from a friend when he butchers his cows (I buy beef from him too, good stuff!).
