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.
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 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