Hi Rafael,
Most likely your structure is incorrect. Your telling LAMMPS to use a lattice parameter of 11.28 Angstroms for your each of the FCC sub-lattices, which
means its going to place the basis atoms for each FCC lattice too far apart ( Recall FCC direct coordinates are (0,0,0),(0.5,0.5,0.0)(0.5,0.0,0.5)(0.0,0.5,0.5)). What you
need to do is use the correct lattice parameter and make the region a multiple of it (i.e. 11.28).
As an FYI, LAMMPS is well suited to directly make compound lattices using the command lattice custom with basis locations supplied.
Hope this helps,
Stefan