I really don't know, but just a thought, perhaps it all too much for her system (even though slowly put on all of it), and you need to simplify her diet. [Or just try a different brand of feed, they definitely are not all created equal.]
Perhaps try changing from all the commercial products to something simple like alfalfa pellets (or cubes, soaked), oats (rolled for ease of digestion) and soaked beet pulp (with maybe a fat supplement like rice bran or even flax for added calories). [Wish I could feed this simply to my couple that need a little extra, but I can't get decent alfalfa pellets locally, my horses won't eat what I can get, so I feed a really plain senior feed (first 5 ingredients: alfalfa meal, beet pulp, barley, oats, soybean meal). Its so nice to know what the ingredients are. Its 12% protein,3% fat and 20% fiber. (some horses may need a little more protein and/or fat than this provides, but it seems to be a pretty good feed).]
Good luck and I'm sure with time, she'll come around and be all you want.