Xcode storyboard assistant editor stopped showing related files.
"Automatic" is selected and "Class" is filled in Identity Inspector.
It was working before, but know it has stopped.
"Auto" or "CounterPart" modes are still woking for other files except StoryBoard.
A few days ago, I tried to update from Xcode 5 to 6, but later on gave up.
Would that have something to do with it?
Found the solution (at least worked for me), it's the same bug that causes you not to be able to create new outlets. You must delete the [DerivedData] folder:
【DerivedData】folder of you project. (This folder may inside your project's folder, or inside ~/Library/Developer/XCode/DerivedData/(your project)/ ) or somewhere else that were setup by you.