I have 4 10 month old hens that I raised from chicks. We waited that long 6-7 months for them to start laying and sure enough, they did. Now, without incident or warning, 3 of them have stopped laying. All are quite healthy, active and seemingly normal. Feathers are beautiful and full with no sign of an upcoming molt. Any idea what the problem could be or what I should look for? All are common good laying breeds.