Since you have already had her teeth checked by the vet, I would consider the possibility that she has ulcers. Horses with ulcers are often fussy eaters. Then you have the cycle of her not eating enough causing more acid in her digestive system irritating her gut and making the ulcers worse. You could talk with your vet about this possibility and if you can, get an internal scope done to confirm. Or you could put her on an ulcer medication and see how she responds. Omeprazole is a the less expensive route, but not all ulcers respond well to it. It only decreases the acid production. Gastroguard is more expensive, but provides a coating to the gut as well as decreases the acid production.
A probiotic is also recommended for unthrifty horses like this mare. A product we have had good results with is Equine Choice.
Grain and processed feeds irritate ulcers. So try replacing her grain ration with soaked beet pulp (the plain kind without molasses added). Alfalfa is good for neutralizing some of the stomach acid as well.