when inserting symbols to a design ensure they are located in a directory that is in the XSCHEM_LIBRARY_PATH list of directories. XSCHEM_LIBRARY_PATH is defined in the xschemrc file as a colon separated list of directories. If you insert a symbol and the leading part of the symbol path is equal to one of the XSCHEM_LIBRARY_PATH directories the symbol is saved in the parent schematic without the absolute path. This makes the design portable. You can also add one directory as [pwd]:
append XSCHEM_LIBRARY_PATH :[pwd]
append XSCHEM_LIBRARY_PATH :$env(PWD)
this makes all symbols included from the current directory portable across all systems.