In addition to what has already been mentioned, I wonder if they might need a bit more protein in their diet. Some grass hays, and therefore some grasses, don't have much protein in them at all. When feeding a horse, I like to have absolutely no less than 10% protein on my easy keepers, but on my hard keepers, I like to keep it up somewhere around 15%-18%.
For that reason, I like alfalfa for keeping weight on a horse. You can get it in pellet or cube form from the feed store. Added protein helps them to build and maintain muscle, which it looks like yours need with the prominent spines and pointy hips.