User blog comment:Theory of everything/Is Nick loosing his Grimm powers?/@comment-1509826-20120422112549

He isn't losing his powers, he can only see them when they get scared, angry, hurt or in any other way stressed, as Geier are so deceteful and cruel, they had nothing to fear from what they thought were ordinary policemen, so they didn't get stressed enough for there true form to be revealed.

Later when the Geier does relise something is wrong, he can clearly see what they truely are, and they can clearly see him. His powers aren't going, he just didn't get the chance to use them.