How long has she been in this new living situation? I wouldn't worry if the change was recent and you do state that she isn't totally at ease with the new situation. When you see her out in the pasture is she eating? It's very easy to get horses in large pastures to come up for grain. She does sound motivated for grain so just get in a pattern of bringing her up at a set time (dusk is a good trigger). Halter her, get her out of the pen so the other horses leave her alone plus you have the opportunity to reinforce some good manners.
Roasted soy beans are a great high quality protein and high fat source for horses.