Weight and an older horse?

Problem with a 25 year old horse that has been steadily losing weight. She is on alpha alpha cubes (about 4cups soaked well) morning and night as well as beat pulp, about 4 cups, soaked, also one scoop of grain, not senior as I have no control over that AM and PM. She also gets about a bale of hay (timothy alpha alpha mix) per 24 hours. there is no gr left on her pasture. She has been on this diet for 3 weeks now (beat pulp and alpha alpha was slowly added), and still no improvement in weight. Is it too early to see any weight gain? What else can her food be supplemented with? Thanks ahead of time!