Maybe try blackberries? They love any organic, fresh, berry they can get their beaks on.. One had a purple blackberry stain on her and that's how we named her!!
I personally don't feed mine milk/dairy but that would be ok, I just meant the artificial stuff... Sorry for wording that terribly. I looked it up and it said the only issue with giving chickens plain yogurt/milk is its a little hard to digest
Artificial preservatives, food dyes such as Blue 40, Red 5, any color with a number after it. My friend's chickens have had bad experiences with yogurt. Sodium benzoate is a common preservative in yogurt that, with enough, can cause cancer/sickness
Maybe it isn't lemon balm? All 12 of mine gobble it up instantly! If you are unsure maybe you could show a picture? I believe you when you say its lemon balm but that is odd...
Update 9 months later incase you are wondering
She is doing really well, laying a little slower, but it took a couple weeks to get her to full health. She is a very happy hen