Hi Juanjo,
If your system is compound not pure metal, then the orientation files are hard to write, because different kinds of atoms are in a same region. For the "fix orient/fcc" command, if there is another kind of atom in the neighbouring atoms of a certain atom, then the orientation filees are hard to write, and the synthetic driving force cann't be imposed on the group of atoms, e.g. cations.
Here 'kind' of atoms is different from 'type' and 'group' of atoms. For a pure system, there is just one kind of atoms but may be different types and groups of atoms.
As i wrote in previous mail, all i wrote above are from my study of this command, some opinions may not be quite right. Hope it wil help you.
L.Young