It sounds like maybe you are trying to hold your left arm too high. It could also be that your partner is letting you hold her arm up instead of using any of her own strength to hold her own arm up. Of course, if you are holding your arm too high, her arm is tired also, which would make her more likely to just let you take the weight of it. Is your hand higher than your shoulder? If so, try lowering your elbow. Your upper arm does not need to be parallel to the floor if you are going for that, and for many (if not most) followers, having it up like that will tire them as well.