same materials not the same...?

Hi, I’m sorry, I’m kind of new to blender.
For a school project, I’m making a restaurant, and right now I’m making windows for it.
The material for the windows are the same, but when I got the material just right for one, the other three are darker.
http://i.imgur.com/Ou6Ri44.png
As you see, the two windows by the door are darker than the one on the right side of the building.
I don’t know what causes this, but I really need to get this project done.

Thanks in advanced!

I see no blend file included with your post !!!

Upload it to http://www.pasteall.org/blend/ and tell us the download link

Also please use the appropriate forum, support questions go into the appropriate support forum
Moved from General Forums / Blender and CG Discussions to Support / Materials and Textures