first, I would have her teeth floated if you havnt yet. Next- and before you go and blow all that money on probiotics etc (personally I think it is generally a waste of money, I really havnt seen any results) FLUSH her with like 2 different kinds of wormers (reason for using 2 is so you kill any kind of worm that might be in her- different wormers have different chemicals that kill different things)
If you havnt done either of those yet that should help A LOT
Next I would suggest adding CANOLA oil to her feed (not corn, flax, rice, etc. canola oil has the most calories per cup of ANY oil and it is one of the only ones with BOTH omega 3 and 6 fatty acids. It is also one of the cheapist) we just take a measuring cup (to regulate but I supose you can just pore it from the bottle) and pour it over the feed. One cup has almost 2000calories.
Next I always recomend the purina omaline- the mare and foal. YES- it says mare and foal. But my guy recomended this to me. it is a 16%. it is great because it has a LOT of protien and calories, but does not make them high or excitable etc like oats or alfalfa may... it also helps build muscles because of the protien. My old horse was on a scoop of the purina omaline and a scoop of beetpulp (and we went through the majpority of a gallon of oil when I first got him) and he went from a dull coat and thin and kind of "back yard looking" to putting on at least 100lbs, got some good muscle definition, a beautiful glossy coat, etc... over a period of the 6 weeks I officially owned him (long story short, he was killed. But when he died he looked the best he had in years im sure) but it has also worked for my best friends 21yo ex-racehorse. With all the horses I have suggested this regiment to or had on it they have all had obvious improvement.
Hope it helps!