Organization Hierarchy Problem: Cannot assign subordinate organizations to an Operating Unit organi

Multi-Org Site using 12.1.3.  We recently created a new operating unit and have been using it for a few months without any issue.  We are not trying to create new organizations and assign them as subordinate organizations to the new operating unit organization.  However, in the Organization Hierarchy form we cannot assign the subordinate organizations to the operating unit organization.  The operating unit organization does NOT have the "Exist in Hierarchy" box checked.  How do we fix this problem?