I have a couple of questions. First, do you get an error about overlapping uv´s?
Secondly: Is that box brush or did you export the cube with uvmapping out of a modeling program?
I don´t know, which result you wanted to have, but I have here two cubes. The right one is a box brush with the normal map dragged on to it and the other left one is a cube out of my modeling software with uvmaps applied to it.
Thanks all for the quick replies, as insideitall said changing the compression settings to TC normalmap fixed the issue. Do we need to make this change every time for normal maps?