I would try adding cocosoya oil to maintain her weight and to keep her coat nice and shiny. This is what I used on my TB last winter when he lost some weight along with some alfalfa cubes and it helped back on the weight very nicely. Also, once the weight had been gained the continuous use of cocosoya oil will help maintain the weight and improves her coat.
I also agree with having her teeth done and trying different types of hay because she might just simply not like the type she has now.
Sorry if I repeated anything I just kinda scanned over the previous posts.