My guess is that you don't use floating-point textures to store the position in, so the value will be clamped to [0..1] for every component in the shader.
Try to define the texture-format in application-code with glTexImage2D( .., .., RGB_32F, .., .., .., .., GL_FLOAT, .. );
That's the trick ... I used GL_RGBA before instead of GL_RGBA32F. Thank you very much, that helped