How are you trying to summon her? The exact method you use can make a lot of difference. Its usually fairly difficult to create things within your field of vision; it helps if you have the appear behind you, through a door, or around a corner or something. Also, have you tried using verbal commands? Those have a lot of power in dreams. Just try saying out loud, "When I turn around, my ex will be standing there."

Do you see this person much during waking life? If so, do a few reality checks every time you see her, and hopefully, this habit will carry over into the dream world for you.