yup i think i understand
You have a directional light, not a positional one. Spotlight settings only work for positional lights. Set the w-component of the position to 1 instead.
Also, don't expect very good results. Lighting is calculated per vertex so you need to define a much higher resolution cube to have some sensible results.
and yes, i didn't expect great results, i'm using a low tesselated cube because, as i said, i want to learn glsl and use fragment shaders to compute direct and indirect lighting
thank you again