ok, i'm having the same issue as OP. I have a custom sky, gloss is white in the alpha of a white spec map, on a dark grey diffuse. The spec intensity is around 3 and the spec sharpness is maxed out and the best reflection i'm getting is:
I place a cube in Marmoset. The diffuse is pure black, the specular is pure white and the alpha within the spec (the gloss map) is also pure white. Together they give my mesh a chrome surface but regardless, I do not have any reflections. I even tried enabling the skybox, thinking reflections were linked to that?