HELP CHICKENS DYING

[whyalla]

I had 18 chickens I hatched from the egg but when they got to 3 weeks old started dying one by one, I am now down to 11 but there is another that doesn't look well.
I have been to the vets and pet shops and asked them what could be happening and they all indicated worms as they think the only other thing it could have been is coccidiosis but the starter mix I have them on has medication against that in it.
I started them on the worm medication a couple days ago but I'm still losing them.

I have bred hundreds of chickens in the past and only lost 2 all up in all the hatches I've done prior to this one.

I should mention this time I bought the eggs off of a friend because I wanted Plymouth Rock chickens

I also have another batch of chickens that hatched 2 days ago in the incubator that I need to put in the brooder that the bad batch were in (I've moved the others to a bigger enclosure as they are older now) but too scared to put them in there invade they catch what ever it is killing the others
I have fully bleached it tho

Any ideas what could be killing them?

And do you think it would be safe to transfer the newly hatched to the brooder now that it has been sanitised?

 

[Eggcessive]

Medicated feed does not always help to prevent coccidiosis, so if they are showing symptoms of cocci (diarrhea or bloody poop, lethargy, or puffed up appearance) then I would treat them with Corid for 5-7 days. A lot of infections can be a result of contamination from eggs or bacteria in an incubator. Starting with clean eggs from healthy chickens is best. If you continue losing chicks after treating for cocci, you might get one necropsied by your state vet.