The import process results with success ( or at least without any error messages about any of the related subform data) when I select the "Do you want to import subform data" option on the data import screen and select the related subform columns on the data relation table. When you return to the report after the import process and look up for those imported data then there isn't anything in the subforms.

What am I doing wrong here?

I can't also find any related help pages on the help guides about this subject.