problem with per fragment lighting

I have a program which lights perfectly a mesh (divided in 3 parts) with a per fragment lighting (with GLSL). The problem is that when i had a plane behind the mesh, this plane and the mesh become black, and we just can see the specular part on the mesh. The light is next to the camera.
Do you know what to do ? and why it do this ?

Maybe giving some code will help.

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