Make sure she fully understands what you are asking. People often try to move too quickly before the dog truly understands what heel means. Look at your own positioning. Our posture plays a significant role in how dogs will respond, things like turning or slouching shoulders can lead to issues with position. Presenting the reward from the right hand (dog on left) can cause dogs to "wrap around" and be out of position. Targeting and Perch work can help with correct position as well as develop hind end awareness. You might try googling Stonnie Dennis, Larry Krohn, Tyler Muto ect... They all have YouTube videos on heeling that are worth watching.