Membership Report (Detail) not showing organizational members
This may be because the details report has a default filter that limits results to those with a contribution status of completed.  Because memberships by relationship do not have a contribution, they would be excluded.  You can click on the Filters tab within the report, and then deleted the Completed value from Contribution status...then re-run the report.

An alternative is to use Find Memberships (directly under the Memberships top level menu), instead of the details report.  That gives you the option to select Primary Members (select Yes), Related Members (select No), or both (do not select anything.)