Thanks for the question. I need some more information for you. What commands exactly are you running to produce this problem?
Is the mesh you're trying to mirror to, the same topology as the mesh you're trying to mirror from?
If they're different, zBuilder will try and do a linear interp between the maps on the two meshes.
The interpolation will sometimes produce maps with all zeros if the muscles are spatially/topolgically different enough.
Can you share the scene you're working with? Or share a scene that produces the same problem?
Thanks, hear from you soon.