Lost Another One Yesterday

I posted yesterday about a sick chicken. I lost her. I have 13 chickens left. Lost one 3 Weeks ago too. They are 6 1/2 months old. The two that died do not have feathers on their butts. I looked a pictures of internal layers that look like they do. But the first one I looked inside and could not find anything odd. I have two more that are missing the feathers on there butt too. From the vent down to between their legs. They are not swollen at all. Both are scratching around eating and drinking fine, do not act sick at all. They have layer pellets, occassionaly cracked corn, cabbage etc. from garden but mostly thats gone now. They do not free range so their runs are sand which I keep pretty clean, Their coops have pine shavings with DE mixed in with it. Same in nest boxes. I can not see any mites or lice on them. And no worms in poop. What left? I did start them with wazine in their water today. Will do that today and tomorrow, then back to plain water. My understanding is I have to throw the eggs out for a week from tomorrow, is this right? Is there anything else I need to do? I am so baffeled! But out of 15 chickens I can't believe I could have that many internal layers. They have all been laying for a while as I would get 15 eggs a day. Yesterday I had eight brown from 11 BRs,my other 3 are EEs so green or blue from them. Their coop is 8 X 16 and runs are 22 X 20, so lots of room. Any other suggestions?