The Attribute Editor does not display relationships for certain selected features.
This issue may be related to the following bug resolved in ArcFM 10.1.
MM46702: Attribute Editor won't display any relationships when there are attribute relationship records referencing invalid objects.
The problem occurs when one feature has an attributed relationship with a feature that no longer exists. In this case the record in the relationship table is referencing an invalid objectID for one of the features.
Remove the attributed relationship record referencing the invalid objectID.