I noticed that, and I was upset about it. I always thought flitwick was odler, like he looked in the first two films. But there is a complete lack of continuity, due to the different director every film. Since the first two films the movies have gotten away from wizard appearance. Most of the time the students arent in their robes, and neither are parents or teachers for that matter. So, i guess they just wanted to make him look more normal, a little less wizardy, just as they have done with everyone else. Although i do not know why!