What's the deal with my Cordon Bleus ? My male has had a bald spot on the nape of his neck for months.?

The vet says he's fine. He sings and dances and courts the hen. It looks like he's going light. I've lost 2 hens one of which had also lost feathers in the same area. One fluffed up on the cage floor. I treated her as if she was egg bound with steam & heat. She recovered only to relaps 2 weeks later. I'm down to one pair and I really hate to loose another one. I feed millet spray, finch mix,boiled egg & brocclli. Should I add Exact to their water?