I'd be concerned with mineral deficiencies. What, if any, minerals do you add to her feed? Or are there minerals and salt offered free choice? Have you ever considered a ration balancer? Just throwing ideas out there, if the only minerals she gets is from a mineral block then she may need more, they typically don't add much by way of needed minerals. I give mine a few different varieties of loose minerals and loose salt as well as a plain white salt lick, all offered free choice. I always just used the mineral blocks and was surprised when I switched to loose, there was a time period of around a week where they were eating a lot of it, consumption leveled out once they were balanced again, and now they only eat it when they need it.
My gelding is an air fern and I still give him his ration balancer, it doesn't add much in calories, but gives him what hay alone can't give him.