I wasn’t very clear before. Are you feeding the same grain or mixed feed as she was being fed before, and the same amount of grain. Are you feeding the same hay she was getting before & the same amount? Not all hays have the same nutritional value. Is she doing more work now, or more? She could need more food if she is being ridden more.
Some horses are like some people and can easily drop weight. You might want to check with the previous owners if this has ever happened before. If she has always been a slow eater, and she is on the same food she was on, have you had her checked for ulcers? How long has she been at your place, and has she settled in well?