Horses are smart, she knows that she can outrun you. My friend Amy has a trick to this with her mare. Chase her to the outside of the pasture, while you stay closer to the inside. You can walk after her and she will be between you and the fence so therefore she will have to walk faster and farther than you. Walk after her, keep her moving. Don't stop when she wants to, jus keep following her. She will get tired of the game and you will be able to walk up to her. Do this every time, its a no-nonsense approach.
UPDATE: Don't use treats, that's bribery and she will become demanding or if she knows you don't have treats (apples, carrots, sugar, mint, grain etc.) she will be unapproachable. Bribery spoils a horse.