Here's what I did...
The first thing I think about was when was the last time that they got wormed? Pretty basic stuff. If the worms are taking all the goodness then they cant possibly produce and egg each day! I have a few different wormers and rotate them so that the worms that do survive one worming get blasted by the next wormer that goes through the chookie. I also use a different system to the usual "replace the drinking water for a day" as I have discovered that even with 34 degree heat in the height of summer, my chookies would rather dehydrate and die than drink chicken wormer. What I usually do is make up a smaller amount of the mixture, dip some bread in it and feed it to the chookies. That way I can see who got the wormer and who didn't. Check out this post for more detail.
Some people put garlic in the chooks water to fight worms - it doesn't matter wether you use natural methods or chemicals - the worms need to go. They will kill your chookies if they get to high enough levels. Wild birds in the chook pen feasting on spilt grain can also reinfect your flock.
Also have a look at the food you are giving your chookies. Again, the same principal applies. If they are not getting enough goodies in they simply cant produce an egg full of goodies for you each day. I was feeding mine the cheapest grain I could find at between $16 and $18. Once I changed to a profession breeder mix, I had egg production double! They don't eat as much of it and so in the end I was paying the same amount for grain over the course of a year. I ended using pellets as I got less waste than the mash - but its down to what works for you and your chookies. Also make sure they are getting greens on a daily basis. The chlorophyll is what gives you the lovely orange yolks and extra bits and bobs too!
If none of this is working try probiotics - especially if you have some dirty bottoms in your flock. Its not a magic cure if you have not sorted out any worm burdens or quality of food issues but if you have then it might just seem like magic! Worming my chookies bought a few more chooks back on the lay, increasing the quality of the food sorted out a few more and increased the quality of the eggs and for the two who still weren't laying, the probiotics worked the miracle. I got mine from the local produce place for about $20 and simply added a spoonful to the feed every morning. If you know which chookies aren't laying separate them out for a small part of the day and make sure they do get some high quality food and some probiotics and see if that makes a difference.
If you cant see why they aren't laying as you have done everything - there is a chance they are but you don't know where. They need a place to lay that is secure, darkish and comfortable. I find if I free range the girls all day, everyday they will find wonderfully impossible places to lay 1/2 a dozen eggs before I figure out where they are! Keep 'em locked up for a few days and have a good look around the pen if you think they are laying but don't see the eggs.
I have had crows figure out where the eggs are and raid the nest before I could get to them each day! I got a huge fright to have a big black crow fly our of the nesting box when I went egg collecting one afternoon! if you don't collect regularly you could be raided by snakes, goannas, crows or even your dog!
Broody chooks don't lay eggs either. They sit on the nest all day everyday for about a month, come out to eat and drink once a day, loose lots of weight and generally look like they have had a life time of abuse! If I notice a chookie sitting around for a few day - and more to the point, there isn't an egg from her, I move her into the other pen for a few days. it can take some of my chookies a week to get over the broodiness and for that week they take an incredible dislike to me! If you don't have two pens, you will need to move her away from the nests somehow to break the broodiness. Maybe an isolation pen? Maybe free range her in the garden? Maybe pop her in an old childs playpen within the chook coop? If she can get back to the nest that she has decided her babies are in, she will stay broody for the month (and possibly a bit more) and you will be without eggs from her for a few months. I used to let my chookies just go through the cycle thinking it was "natural" but it takes a lot out of them and they look tired and sick for such a long time. But since chooks have been bred and mucked around with for centuries now, I'm more interested in keeping mine healthy and happy (and laying!) as there is nothing natural about a chicken with no "husband" trying to hatch babies! So now I move them into another pen and the sorting out of another pecking order gives her something else to think about and usually breaks the brood in 3-5 days. If I let them out of the bottom pen and the broody chook heads straight for the top pen and puts her self in the nesting box, she is still broody! If she is nesting rather than roosting at night she is either unwell or broody also...
I find storms put my chookies off the lay for a day or so. A child getting into their pen and chasing them for extended periods of time can also upset them. Building them a new coop, changing the pen around entirely or even introducing a new chicken to the flock can upset the more sensitive flighty chooks. As the eggs are about 25 hours in production they will lay the morning after the event but sometimes not the few days after. My more stoic, placid chooks only go off the lay if its something physical (worms, food) rather than environmental (Storms new coop).
The more you watch your chookies and get to know them, the sooner you will spot what is wrong and what is not normal for your chook and be able to treat it. An isolation pen that can be moved in and out of the main pens is a real boon in times like this. You can have the chook in with the others and maintain her place in the pecking order and still make sure she gets the right food and or medication that you want to give her.
The higher in the pecking order your chicken is, the healthier it is likely to be. The top chooks get first peck at the food, the best places to sleep and are generally feeling good about themselves. The chookies further down and at the bottom of the ranks are likely to be the ones missing out on the really good food, the treats that you throw in the cage and the medication that you could be dishing out - thus remaining at the bottom of the pile as they are simply not as healthy as the others. the more chooks you have the more likely this is - but it can still happen with three chooks!
Let me know if you have anymore ways that you use to bring your chooks back on the lay when they stop laying. It can be so frustrating to have seemingly healthy happy chooks and simply no eggs!
Green-ness: Depending on what chemicals and what grain you use will depend on your green credintials for this one!
Frugal-ness: There is nothing frugal about owning chickens!!!
Time cost: Lots and lots - but it is soooo worth it!
Skill level: Mainly observation, trial and error.
Fun -ness: Owning chookies is right up there in the fun states! We wouldn't be without them!