He did not mention a specific deficiency. I felt the same as you since I use a quality feed with other whole foods/treats for variety. I believe he prescribed Baytril. I'm not sure if Amoxicillan is broad spectrum......anyone know?
I think you are going to need a broad spectrum antibiotic to properly treat this. That's what ultimately helped my hen. That along with an improved diet. This issue can be caused by malnutrition (typical in broodies) and/or deficiency. Along with the antibiotic we gave her fish oil...