Xystus: I guess Kiria's powers doesn't actually affect the person directly through the eyes, but rather affects the targets feelings.
Erza believes herself to be strong and noble, because she wants to help her friends out in anyway she can (so, in a way she has a strong heart). On the other hand, after the things that happened to Laxus, he believes himself to be weak and thus have a weak heart.
So, Kiria's powers effect might be dependent on the condition of the targets feelings, along with the condition of needing eye contact (aka., the stronger the feelings of the target is, the stronger Kiria's power affects them, while the weaker the feelings of the target is, the weaker her binding is on them).