My OTTB who was super tough to get muscle/weight onto has gained an absurd amount of muscle since starting on Purina Supersport its an amino acid supplement similar to Nutrena Empower (I have friends who like empower but have no experience with it). Make sure they are getting everything they need in their diet but if their diet but if they are then think of adding the amino acids
Make sure her diet has enough protein! Many feeds are deficient if you're feeding below the recommended amount (as most people do with easy keepers). I would look into a ration balancer, it has worked very well for my horse.
Try changing how she lives. Increase paddock time, maybe a more playful herd, less stall time. Even if you ca't ride a little productive lunging (not running in circles, but trotting, cantering, pole work, etc) every day will help. Even hand walking for 20-30 minutes is better than nothing.