When creating a new duct bank through the Underground Facilities Manager interface, the availability field of the new Duct features is not set. When associating conductors and pipes to the ducts, they show as unavailable. When viewing the conduit's configuration, they show as available. Further editing may result in memory errors being thrown, requiring a restart of ArcMap.
This issue is caused by known bug MM55815, marked for inclusion in 10.2.1b SP1.
Until the upcoming fix is available, we recommend that you first create duct configurations within the conduit configuration tool, then create Duct Banks from Conduits within the Underground Facilities Manager interface.