I would also recommend you start putting ACV in their water every day instead of once weekly--1 tablespoonful per gallon because that is a great cocci preventative.
Are all of the sick birds from the new flock? The are several different strains of coccidia--every chicken has some in their gut, but a new group coming to your home may get sick because of it being a different strain of cocci. If these are young birds they may need to be on medicated feed...