There seems to be some confusion between a “field group” and a “field”. A field group contains 1 or more fields. A field is one item in a group.
A field group will be shown even if it contains no fields.
From your response I am going to assume that you mean the field group is not being shown. If this is the case then the first thing I’d check is the “Screen Options” (tab at the top right of the edit screen) and make sure that the check box for the group is ticked.
