3 possible reasons I am thinking of. One is that the process I have been using alot lately is to model my bases in Maya, sculpt in z, then export the sculpted level 1 to Maya for uv-ing and then importing the uvd mesh back to z. As the topology is always the same the detail transfers no problem onto the newly uv'd mesh.... But in the current case there were a couple of verts that somehow got adjusted so there are a couple of issues with how the detail transferred.
Here's a better way to UV your characters.
If you've not UV mapped your critter before you sculpted all the detail, and now need to, and wish to avoid all the hassle of transferring detail and cleaning up any mess. Plus avoid the taxing of your system if you have mutlti-million poly meshes. Then UVMaster is your friend!
Just export the lowest sub-division of your mesh, import it into UVmapping program of choice, UV it, export it back to Z. Now import the mesh and go to Zplugin> UV Master> Copy UVs
Then change tool to your Highpoly mesh, then Zplugin> UV Master> Paste UVs
If you don't have UVmaster plugin, you can get it here:
https://www.pixologic.com/zbrush/features/UV-Master/
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