Question about the do_dssp command

GROMACS version:2022.04

Hello,

I am running the gmx do_dssp command using an xtc file from which i have only removed the water molecules and i get this “warning”:
Not all residues were recognized (9 from 15), the result may be inaccurate!

Should I maybe make an index file for each residue?
I do not understand how can it not recognize all the residues.

Thank you!!