I once had a filly who went off her feed just a couple of months before her big Futurity show, and the vet recommended a product called Ketamalt. It is a suppliment manufactured for cattle, but he said that he found that it incresed appetites in horses. He was right! After just a couple of days that filly would eat up her feed and still licking her lips, would look for more.
I also went the injectable B12 route with a rescue mare I had once, and it worked wonderfully. In less than a month on those B12 shots, that mare went from a dandruffy, sad looking, flat-eyed mess to a shiny-eyed, filled-out mare with a glossy, dappled coat. I wouldn`t have believed it possible, had it not happened in my own barn.