Save the affected Poser run as a .pz3 file. Using a text editor look in the characters' initial "short actors", e.g. as follows. The missing head's short actor's insides may be missing. Copy it back in from a nearby "short actor", and change the end argument of the geomHandlerGeom line to match. Then save the result as ASCII TEXT, preferably under another file name in the same folder.

<b>actor hip:3
        {
        storageOffset 0 0 0
        geomHandlerGeom 13 hip  
        }
actor abdomen:3
        {
        storageOffset 0 0 0
        geomHandlerGeom 13 abdomen  
        }
actor chest:3
        {
        storageOffset 0 0 0
        geomHandlerGeom 13 chest  
        }</b>