Specular map not showing with subsurface scattering shader

Hi, I was wondering if any one might know why my spec map isn't showing up when rendered, I'm using the miss fast skin shader for mental ray. I've applied it to the overall weight and as you can see on my map there's 2 clear white lines on the forehead but specularity is just ignoring it. I've tried mapping it to the prime spec colour but same again. Does anyone know is there something I'm missing?
My UVs are definitely lined up in the right place.

learn an other renderer... mental ray is dead...
nvidia announced it yesterday..

is the alpha connected or the red channel.?
turn on alpha is luminace if the alpha is connected...