Atually, I don't think it is refraction, but transparancy...

Do you have the 'light through transparancy' box ticked?
Are you rendering with GI or IL? There is also a second 'Light through transparancy'

And in the shader there is a box that reads 'no light interaction when fully transparant' when you select the top shader, next to the transparancy channel.

Maybe one of those needs ticking?