Any chance it's boredom?
Does she eat the bits she breaks off? If so, she may need more protein in the diet, or some other micro-nutrient. Even if she's got a good diet, some animals need more of something than those around them do (I need extra magnesium - don't know why, but I can tell if it's low and I'll be eating the same thing hubby and #2 son are eating!)
We tend not to think of this with
chickens, but some individuals also struggle to absorb nutrients even if the diet is good. In
chickens I've heard of their digestive tract being slightly damaged by coccidiosis. In this case, something like a natural
Apple Cider vinegar (diluted in
water) or some version of Kefir might help? If it did help, you'd have to observe to see if once "good bugs" are in the digestive tract they stay and colonize, or if she needs a dose weekly or more or less frequent.