Have you checked the birds over really well for mites? I'd also clean the roosts really well, and either white wash them with a seven dust mixture or spray them with Frontline spray...just in case you've got mites hiding there and coming out at night to feast on your girls.
Plenty of ventilation in the coop? Not getting overly hot in there of the evening?
Any big weather changes recently? Mine seem to lay less when we get rainy weather for days on end.
Just tossing out some ideas...
Also, if your flock has worms, then their egg production would most likely fall off - if you use dropping boards then you can ususally find evidence of roundworm infestations when you go to clean. But if not, it's harder to find. So you might consider a round of worming for your flock (of course there's an egg withdrawal period - the downside)...