There is a convention for the names of corresponding left and right parts, for the symmetrical mechanism to work. Look at the names of the parts in a standard Poser figure. I think the rule is that the corresponding left and right parts must be named l#### and r####, where #### is the same in both parts. E.g in a Poser model of a sumbarine, the symmetricalizing mechanism is unlikely to recognize that portForeHydroplane and stbdForeHydroplane are a symmetrical pair!!