That is because when you modified a character in Zbrush (changing a poly count probably), it is no longer a standard CC character and becomes a non-standard.
Non-standard characters cannot utilize CC clothing items,
transfer skin weight...etc.
You may only
transfer weights from body to cloth outside on CC, like in Blender (or Zbrush probably).
Tweaking/modifying weights for items (rigged to non-standard character elsewhere) is possible in CC though.