something I sometimes forget to do when using normal maps made in zbrush, is to make sure I set the bump node to use as 'tangent space normals' rather than bump. Make sure you have created the normal map with the right settings. And I have been making use of a tip I read which i have found helpful which is to create a cavity map in zbrush and mulitply it over the colour map, to give the normal map a bit of a helping hand like.